Turn
Every moment, even as he viewed the battle-strewn corridors of the Jedi Temple, he believed that his friend could come back. Even as he was told to hunt him down, he prepared an alternate route, with the full intent to turn him back or die trying.
He hid where the only person who knew his location would unwittingly take him there. He watched as she realized the same thing Yoda had discovered, still unwilling to accept it himself.
It was only when he saw that anger, that horrible, terrible anger, turn fully against his former life, that he lost hope.