* Is he aware of what he's doing? Medic says no, and points to schizophrenia. But... are you really gonna trust a man who lost his medical license twelve years ago?

* He doesn't know that it's wrong, but he knows that he's ending lives. The reason RED! Pyro is so feared is because he has no moral code, no reason not to kill.

* Furthermore, it's fun! At least to him. He honestly doesn't understand why people are afraid of him, nor why they seem to be disgusted by his actions.

* He's kinda lonely. Sure, he's constantly surrounded by people, but at the end of the day, there's no one to be with him.