Drink the blood. Blood from flesh. Flay the flesh. Bones under flesh. Break the bones.
Some people are just plain evil.
A path with the evil one is just plain more fun than being a pious innocent person. When someone has evil in their veins, being a monster, commiting vile acts and the like become everyday things. Ah no, I think I shant tell you. To see it would drive you mad, to hear it would soil your soul for eternity and to say it would kill you. Some people just figure it's cool to be bad.