Or cain is the lord of lies?
That guy just spends way to much time with demons to NOT be the lord of lies.
There was always something strange about him in Diablo1. In Diablo2 you 'rescue' him from tristram even though every one else is slaughtered (even wirt

). Now in diablo3 you find him in the depths of a demonic catacomb. He HAS to be the lord of lies.
Tyrael is probably betrayed by Cain and then ends up like Izzy. Diablo and all his brothers are freed somehow and we start back from square 1.