User blog comment:Ralnon/Why didn't Diablo kill the Angiris Council?/@comment-95.155.215.192-20120704180336

My guess is that Diablo wanted the Council to suffer, seeing as he's hated them for tens of thousands of years. If he killed them, and then destroys the Crystal Arch, he'd remove them forever, because there would be no Arch to respawn from! He couldn't have that, so he kept Auriel alive, didn't bother to kill Imperius because the dude was badly wounded, his greatest weapon sundered in one blow and yeah, he had just tossed him around like a ragdoll, so he knew Imperius wasn't really a danger to him.

Diablo had every reason to think he could take his sweet time and do whatever he damned well felt like, and I personally think he just plain didn't WANT to kill the Council. He wanted to gloat, make them see their precious High Heavens corrupted, and then have them tortured for the rest of eternity.

It's definitely not an unreasonable assumption to make, considering all those angels they had suffering for their amusement, and the fact that Mephisto himself takes great pleasure in tormenting Inarius, and even agreed to a treaty in return for that particular angel.