I think people are misunderstanding "the hunger" thing. It's not a hunger for brains or wanting more powers, it's for wanting more and more knowledge of how everything works and trying to fix them or yourself to make you better.
Doesn't make Sylar any less of a horrible person then he is, it's still no excuse to kill innocent people. It's like a drug - when you're hooked on it, you'll do whatever you need to do to get that next fix even if it means harming other people. He just uses it to make himself feel better and Peter has totally been sucked into it.
It also doesn't help that you've had people around Sylar feeding into his ego - i.e. his adopted mother, and now Angela.