Oooh, here's a nasty thought. What if it's the Sage of Six Paths's son? The one who was spurned and paved the way for the Uchiha Clan? CENTURIES-OLD HATRED. Plus his stated goal is to revive the Ten Tails and become its Jinchuuriki, like the Sage was.

Maybe it's a whole plan to recoup the power he always believed was his.