User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-5174994-20140806154246/@comment-413232-20140806211624

So, he deserted the Assassin for the Templars, this explain why his brothers tried to kill him. They tried to kill him not because he was leaving but because he was leaving for the Templars, this make more sense.