I'm seeing a lot of revenge, anti hero, but for me is far from that. The whole idea of revenge is to put the status quo back to 0, which in this case is not. My reason for this assessment is that the hero put the current world in a state of pace. His work had been fulfilled, but the story is not about that, instead is the rebirth of evil portrayed by the descend into madness of the protagonist. In other words, he wants to undo what he achieved he wants to put everything into chaos. It is poetical in a way, because at the moment of return he transform in an oposite of himself.
Again he knows that is impossible to get anything back, not pace, not justice , not retribution, just chaos and destruction.
He himself said it, he must restrain not to kill so he can savor the pain as long as he can.
Really fucked up, but really amazing