I interpreted it to mean that The Doctor has had enough. He's just spent 2 billion years in a walking nightmare, enabled by the traditions of his own people, who have frequently behaved as complete knobheads, and he's fed up with it all. He's finally home, after having seen it effectively destroyed by his own hand, and he's going to do something about it. He's decided that since he's already done the things that The Hybrid is prophecised to do, he may as well be The Hybrid.