What I have heard and what I firmly believe from playing past diablo games and reading the stories and such is that Tyrael, Hadriel, and Izual end up being the actual three prime evils. The worldstone now destroyed, Heaven and Hell collide into earth forming Tyraels new realm. Tyrael seems to have just used the hero to rid the earth of Diablo Mephisto and Baal and give him free access to the worldstone so he could destroy it.

Reasons why I believe this:
Tyrael says he has little power in the realm where the hero is, that is why he wouldn't kill diablo mephisto and baal himself, so he gets the hero to do it and give him the ability to make his own realm where he can have his power.

Tyrael says something about how he went against the council or something(I think it had to do with the soulstones.)

Tyrael is a fucking dark angel with a big ass sword.

Same for Hadriel and Izual.

More things I can't think of.

Go ahead and tear it apart Senesia, I know you're dying to.