I actually do buy his self-imposed exile though. He said it himself, he destroys everything he loves. I don't think he was equipped emotionally to deal with Deb's death. From the beginning she has been his strongest tie to normalcy. Like he says in the narration, first he wanted nothing more than to feel...now he just wants it to stop. To experience something like the loss of Deb when he was at his most "human" and know deep down that it's his fault...it's too much for him. He realizes now that he can never really change. And he doesn't want to continue being Harrison's dad if he's an empty shell of himself and has the need to kill again. There's still a loving, human part of him deep down that loves Harrison and Hannah, but it's buried in a sea of darkness that he knows he'll never be able to escape. He doesn't want to inflict it on them.