Our powers work by tapping into our own endurance. It's like turning potential energy into kinetic energy. Human souls are all potential energy - that's why consuming them gives you a boost. It's not stealing so much as just killing and eating them - it's not like the human's alive after to worry about it.

I've never done it... my roommate's done it once or twice but it didn't help her much. Half-bloods have it easier in the 'blending in' part but they're kind of limited in the power department.

[... He sighs.]

This is depressing, sorry.