When Snape was in the company of Voldemort, why did he not just use Avada Kedavra on him?
Voldemort's body would again be destroyed, and he'd become a soul again, and we know that in this form, Voldemort could easily be dealt with (hence why he hid himself in Albania's forests), when compared to a fully corporeal Voldemort.
While he was a soul and trying to get another body, you could spend that time killing the rest of his horcruxes.
Well you are of course right to suggest in your question that robbing Voldemort of his mortal body isn't a bad plan; he could not be a general, he could not set about acquiring the Elder Wand, he could not lay his plans.
But it would not kill him, it would just leave him bodiless once more. But this time, this time his Death Eaters will not believe him gone, this time Wormtail knows what to do to resurrect him. Is it worth risking Snape's life for this minor delaying tactic?
Well, clearly Dumbledore and Snape don't think so, and I would agree with them. Snape is the spy, the double agent, the man on the inside, without him, Dumbledore, Harry and the entire Order are much worse off in the long game.
That's one part of it. But a second part of it is that it is not actually all that easy to kill a man. It's harder than you imagine. The enormity of the act. Making sure your aim is true, choosing your time, your weapon, making your escape. This is why real life assassinations are very carefully planned and many of them fail. Think of Castro and the veritable legendarium that his built up around the attempts on his life. Could Severus Snape presume to surprise Voldemort alone and just fire off Avada Kedavra and get it over that quickly? Not obviously. Voldemort is not very often in such a vulnerable position.
[S]he had no wand upon her either ... how stupid they were, and how trusting, thinking that their safety lay in friends, that weapons could be discarded even for moments ...
Harry Potter and the Deathly Hallows - p.281 - Bloomsbury - Chapter 17, Bathilda's Secret
[I]t had been a grave mistake to trust Bellatrix and Malfoy: didn't their stupidity and carelessness prove how unwise it was, ever, to trust?
Harry Potter and the Deathly Hallows - pp.444-5 - Bloomsbury - Chapter 24, The Final Hiding Place
'...You will hear many of his Death Eaters claiming that they are in his confidence, that they alone are close to him, even understand him. They are deluded. Lord Voldemort has never had a friend, nor do I believe that he has ever wanted one.'
Harry Potter and the Half-Blood Prince - p.260 - Bloomsbury - Chapter 13, The Secret Riddle
Once he has his body back, he does not have around him trusted assistants who could sneak up and kill him in his sleep.
And what if he should be vulnerable for a second? The creak of a floorboard, Voldemort turns around, the curse just misses, and then what does Snape do? Well, I'll tell you what he does, he dies, that's what he does :P
But perhaps you're not convinced, so I'll add one final thing. Tom Riddle can read minds.
'[...] The Dark Lord is highly skilled at Legilimency -'
'What's that? Sir?'
'It is the ability to extract feelings and memories from another person's mind -'
Harry Potter and the Order of the Phoenix - p.468 - Bloomsbury - Chapter 24, Occlumency
And Tom Riddle is so accomplished that he can do this without even looking at the person.
'[...] And I'll tell you this, too,' he added, on a sudden inspiration, 'my wife knows I'm up here, and if I don't come back -'
'You have no wife,' said the cold voice, very quietly. 'Nobody knows you are here. You told nobody that you were coming. Do not lie to Lord Voldemort, Muggle, for he knows ... he always knows ...'
'Is that right?' said Frank roughly. 'Lord, is it? Well, I don't think much of your manners, my Lord. Turn round and face me like a man, why don't you?'
Harry Potter and the Goblet of Fire - pp.18-9 - Bloomsbury - Chapter 1, The Riddle House
Now, it's true, Snape can close his mind. But can he kill his own master, Tom Riddle, without Voldemort noticing? I wouldn't bet my life on it? And recall, Avada Kedavra requires a strong bit of magic behind it. It seems to be perhaps the only curse that cannot be cast non-verbally. You can't just kill someone like that.
'Avada Kedavra's a curse that needs a powerful bit of magic behind it - you could all get your wands out now and point them at me and say the words, and I doubt I'd get so much as a nose-bleed. But that doesn't matter. I'm not here to teach you how to do it.'
Harry Potter and the Goblet of Fire - p.192 - Bloomsbury - Chapter 14, The Unforgivable Curses
'Never used an Unforgivable Curse before, have you, boy?' she yelled. She had abandoned her baby voice now. 'You need to mean them, Potter! You need to really want to cause pain - to enjoy it - righteous anger won't hurt me for long - I'll show you how it is done, shall I?
Harry Potter and the Order of the Phoenix - p.715 - Bloomsbury - Chapter 36, The Only One He Ever Feared
And these necessary emotions that you must summon up can be read by a Legilimens.
You could try and stun him first, I suppose, or something like that. You could try. But your fear, your nervous excitement. Maybe Snape is the one person who could keep it down, but surely Tom Riddle is the one person who could notice. And it's not as if he spends much time in the company of Severus Snape. And I'm sure he's never wandless, never unable to notice Snape attempting to draw his wand, never with his guard down.
Which means that Snape assassinating Voldemort would never be the plan - it's too high risk. Which means that he wouldn't be looking for opportunities. Maybe, in all the years when Voldemort had a body there were times when a man looking for an opportunity to kill Tom Riddle, might have been able to do it, just. But Snape would not have been looking for such an opportunity. He would not have been trying. So he would not have noticed these moments and if he did, by the time he'd thought about it, they would have passed. Voldemort would have faced him once more, would have moved away, would have summoned another Death Eater, or whatever.
‘Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort’s soul was blasted apart from the whole, and latched itself on to the only living soul left in that collapsing building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort’s mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to, and protected by Harry, Lord Voldemort cannot die.’ Harry seemed to be watching the two men from one end of a long tunnel, they were so far away from him, their voices echoing strangely in his ears.
‘So the boy … the boy must die?’ asked Snape, quite calmly.
‘And Voldemort himself must do it, Severus. That is essential.’ -Deathly Hallows
So he knows that anything he does will merely slow Voldemort down.
Now previously it took Voldemort some 12 years before he regained his body, however, we know that with someone to help him creating a new body is simple.
‘I remember only forcing myself, sleeplessly, endlessly, second by second, to exist ... I settled in a faraway place, in a forest, and I waited ... surely, one of my faithful Death Eaters would try and find me ... one of them would come and per- form the magic I could not, to restore me to a body ... but I waited in vain ...’
The Death Eaters never went to help him, and all thought him dead the first time. But now the majority of the death eaters are standing here seeing him come back from the dead, witnessing Voldemort explain how to bring him back again in the graveyard scene.
But I was willing to embrace mortal life again, before chasing immortal. I set my sights lower ... I would set- tle for my old body back again, and my old strength.
‘I knew that to achieve this – it is an old piece of Dark Magic, the potion that revived me tonight – I would need three powerful ingredients. Well, one of them was already at hand, was it not, Wormtail? Flesh given by a servant ... ‘My father’s bone, naturally, meant that we would have to come here, where he was buried. But the blood of a foe ... Wormtail would have had me use any wizard, would you not, Wormtail? Any wizard who had hated me ... as so many of them still do. But I knew the one I must use, if I was to rise again, more powerful than I had been when I had fallen. I wanted Harry Potter’s blood. I wanted the blood of the one who had stripped me of power thirteen years ago, for the lin- gering protection his mother once gave him, would then reside in my veins, too ... -Goblet of fire
So Snape killing Voldemort at any point from book 4 on at most buys the trio however long it takes to brew the potion, at a cost of potentially his life, and loosing the ability to spy on their plans, as well as potentially messing up Dumbledore's vague plan he was shared in which Voldemort has to kill Harry.
In fact killing Voldemort and then having Voldemort use someone else's blood to make a new (3rd) body, would lose the protection Lily provided that lived on in Harry's blood, the protection that kept Harry alive in the Forbidden Forest
"Precisely!" said Dumbledore. "He took your blood and rebuilt his living body with it! Your blood in his veins, Harry, Lily's protection inside both of you! He tethered you to life while he lives!" -Deathly Hollows
This tethering of life between the two of them is essential to allow Harry to have Voldemort kill off the piece of his soul inside of Harry, without Harry also dying.
Snape is not aware of all of the things going on, with Harry and the horcruxes, nor the connection between harry and voldemort due to the lilys sacrifice, But Snape does trust Dumbledore utterly, and Dumbledore has made it clear that Voldemort has to kill Harry.
Yes, destroying Voldemort's main body would have bought some time, but the main reason Snape didn't do it: It was not easy to hit the Dark Lord with a killing curse.
Voldemort was one of the most powerful wizards of all times. Even Dumbledore failed to do it with Elder Wand. Last time, Voldemort lost his body due to lack of knowledge. Although Snape didn't know about Elder Wand, Dumbledore would never have suggested Snape to attempt such a foolish thing. Voldemort was an exceptional wizard, Snape wasn't.
Voldemort trusted nobody. He didn't use to sleep with Snape and it was his requirement to remain vigilant all the time because he used to be so mean even to Death Eaters. You raise a wand and you won't have time to contemplate the spell before you are dead.
Other than that, the air around Voldemort smelt full of terror. It'd have required a great deal of bravery to overcome the fear. Assuming Snape could overcome such fear, the probability that he could succeed was close to nil. And, any failed attempt would just blow out his cover. So, it was too risky. Not to mention, even if he would have succeeded, it was going to blow out his cover anyway. Dumbledore wanted Snape to be undercover agent until the end of the war and destruction of Voldemort's main body wouldn't mean the end of the war.
Consider: Albus Percival Wulfric Brian Dumbledore is indisputably the most powerful wizard of his generation. This is the wizard who defeated one of the most dangerous Dark Wizards of all time. This is the wizard who twice founded the Order of the Phoenix to hold back the Death Eaters, who lead one side in both the First Wizarding War and the Second Wizarding War and survived. This is the wizard who spent two decades fighting Voldemort, who remembers him from back when he was Tom Riddle, who has built up an extensive record of his life and so probably knows him better any other wizard, alive or dead.
Dumbledore fought Voldemort in the Ministry of Magic (in Harry Potter and the Order of the Phoenix). He fought side-by-side with the The Chosen One. He fought with reputedly the most powerful magic wand ever created and possessed the Resurrection Stone; between Harry and him, as they fought Voldemort, they possessed all three of the Deathly Hallows. They were even assisted by an immortal songbird.
They lost.
Could Snape have gotten out an Avada Kedavra before Voldemort, now carrying (if not possessing) the Elder Wand, stopped his attack and killed him instead? To misquote Chris Sims, "Not on [Snape's] best day, not on [Voldemort's] worst". If killing Voldemort had been that easy, one of his nastier "allies" would have turned on his earlier, or he would have been killed by one of the very skilled warriors (Mad-Eye Moody! Kingsley Shacklebolt! Regulus Black!) who took him on across two Wizarding Wars. None of them got close. Dumbledore didn't get close.
Literally the only person to ever hit Voldemort with an Avada Kedavra that had any effect at all was Voldemort himself.
Snape wouldn't've had a chance. His best strategy was to try and move the pieces (in particular, as @Jyrki Lahtonen pointed out, the last horcrux, Harry himself) into place and hope things turned out successfully. But attacking Voldemort would just have meant a pointless and probably extremely painful trip to the grave for Snape.
A few reasons I can think of - but I need to find quotes; I'm not near my books at the moment.
One of the problems with "kill his body, then work on the horcruxes" is that it's not clear that Snape, or for that matter anyone short of Harry, would be able to kill Voldemort's body—the prophecy appears to indicate that. Unless you have evidence that Snape would have that capability?
In addition, one advantage Harry seems to have had from his involuntary communications with Voldemort was some information about where the horcruxes were (as I remember, the goblet was one of these items). With Voldemort dead, this source of information disappears, and the likelihood of finding all the horcruxes (and thus accomplishing the plan you propose) gets smaller.
Finally, suppose that Voldemort's body is somehow killed. There's no guarantee that the amount of time he is out of commission will be the amount of time necessary to destroy the horcruxes, and crucially the longer we have to spend waiting for his return (even assuming the horcruxes are destroyed) the greater the chance that something will have happened to Harry which might prevent him from being able to finish off Voldemort. If you have an infallible weapon, you want to use it as quickly as possible while you still have it.
Love
Dumbledore was a compassionate man. He knew there was a chance of redemption (only slight) and he had to give Voldemort that chance.
Harry was also compassionate, and gave Voldemort one last chance to redeem himself in their final duel at the Battle of Hogwarts
‘It’s your one last chance,’ said Harry, ‘it’s all you’ve got left … I’ve seen what you’ll be otherwise … be a man … try … try for some remorse …’ ‘You dare –?’ said Voldemort again. ‘Yes, I dare,’ said Harry, ‘because Dumbledore’s last plan hasn’t backfired on me at all. It’s backfired on you, Riddle.’
It was the only way to save Harry
A piece of Voldemort's soul resided in Harry. Voldemort could not die until this was gone. The only way this could be safely removed was by Harry accepting he was going to die. That magic (somehow, I think it's linked with Voldemort using Harry's blood to regenerate) allowed Voldemort's killing curse to affect only the soul fragment and not Harry.
Killing Voldemort's body and then destroying his horcruxes would have meant having to kill Harry to make sure he was gone.
I may be very wrong, but the way I interpreted the endgame in Deathly Hallows is that it turned out that
Harry (or at least the part of his brain connected to the scar) was one of the horcruxes.
At least it made sense to me (a horcrux had to be there when you kill somebody, and Harry was the closest thing to Lily Potter, when Voldemort killed her).
So, like it or lump it, to permanently defeat Voldemort
Harry had to be "killed" as well.
And the only way to bring that about was to "riddle" Voldemort into destroying his last horcrux himself.
Capitalizing on the fact that Voldemort didn't realize that Harry was also a horcrux.
Therefore, anything Snape could bring about would yield temporary gains at best.
Pragmatically - because the series is about Harry Potter, and without him being the focus (and Gryffindor being the house of bravery) then it would have made a much less satisfying story, may not have turned into a movie franchise, and encouraged the growth in YA fantasy literature. It may make a good idea for some fan fiction, but questioning character choices for the prime work (particularly if it is successful) is just challenging the story that the author is weaving. But yes - there is now heaps more material for fan fic than would otherwise have been the case ;-)
