He doesn't know who he is anymore. First he was Gabriel then he was Sylar then he was Nathan then he was Sylar and now he is broken (shattered) and all his names are falling through the cracks.
Gabriel is stamped all over his past. Every wake up call, every attendance list, every birthday cake, every single 'I love you' his mother could provide. Gabriel Gray was the man that ran headlong into the nightmare that he is trying harder than anything to forget. He cannot be Gabriel.
Sylar is less of a name than a symbol of fear, the sheer definition of monster. Sylar is that huge, terrifying mass of memories, murder upon murder etched in blood, dripping down his arm. Sylar is that word, that awful reminder, hurled like a curse (and usually accompanied by one) by Peter as a constant reminder of what he did. He cannot be Sylar.
Nathan is dead, but he emerges in memories that swim to his eyes whenever he sees Peter's face. Nathan feels some of his pain every time Peter bites back at him for recounting one. Nathan is the reason why it's taking Peter years and years of each other's exclusive company to forgive him. He cannot be Nathan.
Then who am I?
The steady, dull, thud of the sledgehammer pauses. Peter turns around and gives him a look like: 'What the fuck are you doing having an identity crisis when we have to bust out of here and save my deaf girlfriend, thus saving whole damn world (whoa déjà vu) again or whatever it is I won't stop going on about.' Of course Peter doesn't get it, he's always been Peter fucking Petrelli, the very definition of a hero complex, forever embarking on some sort of completely illogical quest to save the world. Of course Peter doesn't understand what he's going through, he shouldn't even expect him to care, and of course he doesn't. People don't care about people they hate. They especially don't care about people who have killed them. They really especially don't care about people that killed their brother.
But then something in Peter's eyes soften in a way that jumpstarts his heart and completely disproves his line of thought. And he's reminded for the umpteenth time that-
You're everything good in the world.
Peter opens his mouth but closes it, maybe out of shock, maybe out of sadness, maybe out of understanding that it was not his brother who spoke, but the shattered man in front of him, whoever that man is, and that he means it, he really does.
And Peter's look, Peter's silence, Peter's maybe-understanding is enough to give him a purpose, a reason, a someone to piece himself up into a person for.