Why don't Death Eaters always use Avada Kedavra?

Question

When the Death Eaters attacked Hogwarts, why did the Death Eaters not just use Avada Kedavra and kill everyone?

Answer 1

The main reason why they didn't use Avada Kedavra is that they are attacking a school full of children. Although Death Eaters are what we consider "evil", not all of them are as bent on evil as Voldemort. We know of at least 4 (Crabbe, Goyle, and the Malfoys) of the Death Eaters had children of their own in the castle. In fact even Voldemort expresses a desire to not "shed magical blood" during the battle.

"If you continue to resist me, you will all die, one by one. I do not wish this to happen. Every drop of magical blood spilled is a loss and a waste."

The intent of the Death Eaters is to take control of the school, and capture Harry Potter, they were not intent on killing everyone.

If you want to read up on some (speculative) logic on the topic of spell casting these answers provide excellent insight into spell casting, which would provide a reasonable explanation if the Death Eaters did want to kill everyone why they still wouldn't always use Avada Kedavra.

Answer 2

Avada Kedavra has no counter-spell, however, I have always assumed that the ultimate defense against all spells is them not hitting you.

This opens the window for better combat spells than Avada Kedavra which have (a) greater accuracy, (b) greater velocity, (c) greater area of effect or (d) have lower "charge time". It is quite difficult to assess accuracy or area of effect but we can get velocity and casting time from the films:

Higher velocity spells

During many battles the spells being used by the Death Eaters appear extremely high velocity (aka undodgable); it is in fact not possible to see them having any travel time at all. See 7:04 in this video of Harry Potter duels.

On the other hand Avada Kedavra spells appear to have moderate velocity over long distances. As can be seen in this compilation of Avada Kedavra spells.

Quicker casting spells

From the same video it seems certains spells can be cast near instantly (such as non verbal spells) and Avada Kedavra is not one of them. So again Avada Kedavra may not be an ideal combat spell.

Answer 3

Doesn’t Voldemort say himself that “every drop of wizarding blood spilled is a loss”? They do not want to kill everyone as that weakens the magical community, there is no pleasure in being a king of rubble.

I would guess (this is not supported by anything other than my guess) that Voldemort would eventually want to control the world and either get rid of muggles or, at least, rule them. This is much easier to do with an army.

When they ramp up their aggression level, after the continual refusal to hand over Potter, they probably do start casting the spell more but it is still in their interest to actually kill as few magic users as possible as they might be able to use them later - even if they have to control their minds to do it.

I would also hypothesize that the Killing Curse might be more difficult to cast perhaps taking more time or energy and thus one does not always have the opportunity to cast it in a fast flowing fight where staying alive yourself is more important than trying to kill someone.

Answer 4

They wanted a pure-blood society - they couldn’t kill everyone.

The Dark Lord and the Death Eaters were trying to establish a pure-blood society, where noble-born wizards ruled over Mudbloods and Muggles. Their main goal wasn’t slaughter. They needed wizards to build their society with, and if they just killed everyone they wouldn’t have very many. They especially didn’t want to kill off pure-bloods - pure-bloods were rare and valued. Neville told Harry he knew that Dumbledore’s Army was safe from being killed because the Carrows didn’t want to spill pure blood.

“Neville shrugged. ‘Doesn’t matter. They don’t want to spill too much pure blood, so they’ll torture us a bit if we’re mouthy but they won’t actually kill us.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 29 (The Lost Diadem)}}

The Dark Lord realizes that he needs to have enough wizards to build a new wizard society, and tries not to kill needlessly, though he was certainly willing to kill when he thought it necessary. He offers those fighting him at Hogwarts a chance to join him. He makes the effort to get them to follow him and join out of fear for their lives, rather than just killing them, and losing the opportunity to get more wizards to join him.

“The battle is won. You have lost half of your fighters. My Death Eaters outnumber you and the Boy Who Lived is finished. There must be no more war. Anyone who continues to resist, man, woman or child, will be slaughtered, as will every member of their family. Come out of the castle, now, kneel before me, and you shall be spared. Your parents and children, your brothers and sisters will live, and be forgiven, and you will join me in the new world we shall build together.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

The Death Eaters and the Dark Lord will gladly kill people who they know will remain against them, but they would rather have allies than corpses. The Dark Lord is often willing to offer the chance to join him before killing. He even offered Neville Longbottom, who he knew was giving the Carrows a lot of trouble at Hogwarts, a chance to join him instead, despite his clear history of stubborn disobedience.

“But you are a pure-blood, aren’t you, my brave boy?’ Voldemort asked Neville, who stood facing him, his empty hands curled in fists.

‘So what if I am?’ said Neville loudly.

‘You show spirit, and bravery, and you come of noble stock. You will make a very valuable Death Eater. We need your kind, Neville Longbottom.”
_{^{- Harry Potter and the Deathly Hallows, Chapter 36 (The Flaw in the Plan)}}

The Death Eaters and the Dark Lord don’t simply cast Avada Kedavra on everyone all the time because it doesn’t suit their long-term goal of ruling the world and having a wizard-run society.

Answer 5

Some of the answer is given by Slughorn in the pensieve while he and Tom Riddle discuss the implementation of Horcruxes.

Killing is an act against nature and changes the killer as well as the killed. Even if you're not placing your soul into objects you experience a "splitting of your soul" when you kill. If you're not prepared to stash that lost soul part into an object then it's presumably gone forever. You pay a price for using the killing curse. So each Death Eater would only do it if they're willing to pay the price on a personal individual level, and most would not do so frequently. Only a few of the Death Eaters are psychopaths and don't care: Voldemort, Bellatrix, etc.