The Lonious Monk
Celestial Souljah
Isayama wrote the ending to AoT first them wrote the rest of the story. It's not really a heel turn, he has always been like this but it took awhile for the viewer to notice he was always the villain.
Im still a Yargerist, if the rest of the world dies then the Ymirs children can rebuild and live peacefully. However I think Eren has another objective than whats presented.
He may have always meant to be the villain, but in the story, he was not a villain at the beginning. He may have had qualities that could have led him down that path, but in practice the only thing he did before the found out about everything outside of the island was try to help his friends and the other people on the island. If he had never learned the truth of their situation, he probably would have just lived and eventually died as a hero in the fight against the Titans.