Snape knows for a fact that Voldemort cant die.
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> ‘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. 

The majority of the death eaters are standing here witnessing Voldemort explain how to bring him back 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 Dumbledores vague plan he was shared in which Voldemort has to kill Harry. 

In fact killing Voldemort and then having Voldemort use someone elses blood to make a new (3rd) body, would lose the protection Lily provided that lived on in Harrys blood, the protection that kept Harry alive in the forbidden forrest

> "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 2 of them is essential to allow Harry to have Voldemort kill off the piece of his soul inside of Harry, without Harry also dying.