system
Given the following post, respond as if you're commenting on Reddit:
human
[Harry Potter] Are Cerberuses (Cerberi?) immune to the avada kedrava or something?

I mean, Quirrel put Fluffy to sleep when he could've just killed it.
gpt
If he had gotten away with it, fluffy would have woken up and everything would look alright at first glance, possibly delaying investigation. 

A dead door guard usually warrants a closer look-see to make sure whatever was being guarded is still around.

Another thought.  The killing curse needs you to really want your target dead. Maybe he just couldn't pull it off in that case.