Yeah. Maybe Tyrael has a hidden agenda
Like... he is following you everywhere, giving you advice, telling who to bring down.
You kill Inarius and Lilith... and bang...
... in the end Tyrael come down to see you with his gut hanging out of his abdomen and half his jaw missing and tell you you did a great but messy job and that now he rule the legion of chaos and he was bad from the beginning and the prime evil won't be happy and stuffs and he send you out in a wormhole...
And then we will have to wait years before Blizzard tie the loose ends of the story in a subscription only sequel in World of Diablo.
But honestly, so much people expect Tyrael to turn bad it wouldn't be that much of a twist end. More like a "'told you so"Â flame fuel.
And that's the end of that!
:p