I really fucking love this. This is like heroes remorse. We’ve traveled with Jack up to this point: met him when he was just a flippant con man, saw when he sacrificed himself for Rose and the Doctor, saw the moment became immortal, met him again when he was leader of Torchwood trying to follow the example he thought the Doctor set, and at the end of it all he’s at the end of CoE, he looks back at his sacrifices, and he says enough.
From hero, right back to coward.
And Jack is tired. He doesn’t see the point any more. He believes he’s a monster, that he will never be good enough. Believes he will always lose the people he loves, and probably because of his own actions. And so he runs, a man with literally all the time in the universe.