system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Why don't Death Eaters open their fights with the killing curse every time?

I'm looking back at the Battle of the Department of Mysteries in the film, where Harry's friends and then the Order of the Phoenix get tangled up in extended duels with Death Eaters, and the sequence ends when Bellatrix casts the Killing Curse on Sirius.

What was stopping her, or any of them, from using it right at the start? They're already deadly wanted criminals, and they probably don't fear the threat of a trip to Azkaban anymore because half of them have already been there. If the Killing Curse is supposed to pierce through defensive magic, why would they bother with anything else?

Obviously they want to leave *Harry* alive to save him for death at Voldemort's hand alone, but Lucius was explicitly threatening to kill the others, and they have no other real reason to spare them.
gpt
Voldemort didn't want to kill wizards. He wanted to rule them. He actually hated killing wizards, especially true-bloods. That was obviously carried over by his followers. Sirius represented a true-blood family that, except for him, were followers of Voldemort. They would prefer he join them instead of killing him.

Bellatrix is just a bit insane, however, and hated Sirius with a passion.