Prisoner of Azkaban is still probably the best, Alfonso Cuaron's direction made it.
IIRC if you kill something defenceless, yes it does. Or just killing something defenceless in general. Voldermort's soul was cursed when he/quirrel killed the unicorn.
Hang on...
(a quick Google later...)
"The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something so pure and defenceless to save yourself and you will have but a half life, a cursed life, from the moment the blood touches your lips"
So there we go. Isn't the imagination a remarkable thing