Other way around. Cain is supposed to have been the first Templar (or at least his mark became the symbol of the Templars), and he killed Abel to steal the Apple.
It's strange that the first assasination in the world (according to bible ofc) would be carried out by a templar, not the assassin.
Some may not know that Abel and Cain had a sister, Set. I'd love to see her in the game.