User blog comment:Ralnon/Why didn't Diablo kill the Angiris Council?/@comment-2154860-20120527231745

Perhaps because Diablo had such an easy time defeating Imperius that they decided to keep them around to torture? The armies of Hell ravaged the High Heavens quite well and Imperius certainly was little more than a ragdoll for the Lord of Terror to throw around and might have been too overconfident after razing Heaven.

To have Tyrael join the Nephalem? Perhaps it was because he knew the forces of Evil were more powerful than the Archangels had expected and that there was a good possibility he'd be slain outright?

I don't have canon on my side so perhaps you should get Chris Metzen's email and ask him, since he'd be the authority on it.