I agree, the whole traitor was a really interesting addition to his character. It shows that he believe the Templars to be right, not just 'oh yeah, I kinda just decided to become one, ya know?'. Besides, it's not as though this is the first time Ubi did the 'traitor Assassin' thing - Lucy, some Italian guy, etc. My headcannon is that he chose to become a Templar after Birch swooped in and persuaded him.