User blog comment:Ralnon/Why didn't Diablo kill the Angiris Council?/@comment-5136207-20120603233300

When he captured Auriel, rakanoth got rid of all the hope from the High Heavens. When Tyrael was all dreary at the beginning of act 4, he had no hope. So i pretty much took away the angel's fighting spirit. That made it easier for the demonic legion to fight the angels.

For Imperius, I think Diablo was going to kill him after he stabbed him, but Imperius escaped just in time.

And Itherael, im not sure, maybe they just wanted the Scroll of Fate, but itherael may have not wanted to give it to him