Because he can't live in a world without her... and because he hates the system that caused her death and wants to change it. In his mind, what he's doing is the best possible thing for all of us, because he thinks he can create a perfect world where everyone will be happy and where Rin will still be alive. He's...

[ Kakashi pauses for a moment, trying to find the right word. ]

...broken. And I'm responsible for it.