In Philosopher's Stone Quirrell goes down the trapdoor, gets through all the defences, and comes to the mirror.
I've said on answers before that I reckon the defences weren't meant to stop anyone at all, merely slow them down so Dumbledore could go take a look. Nonetheless, to get to the Stone you have to pass a giant three-headed dog, and a troll.
Quirrell took the time to figure out how to get past Fluffy, and he 'had a way with trolls': he put Fluffy asleep and knocked out the troll (with his own club, I believe the implication was) (not sure how much of a 'way' Quirrell has with trolls in this case: you hardly need to be diplomatic to knock someone unconscious).
But why? Why put Fluffy asleep? Why knock out the troll? Why not just kill them both and stroll through?
I don't think it's because the creatures cannot be killed by magical means. There are a ton of spells that can kill, and many use 'natural' power (incendio causes fire, as opposed to avada kedavra, I mean).
What were Quirrell and Voldemort's motives for not killing Fluffy and the troll?