User blog comment:Ralnon/Why didn't Diablo kill the Angiris Council?/@comment-3087036-20120618135120

Typical villainous pride. While Diablo by himself would probably have done the smart thing and gotten rid of them, you half to remember that he absorbed the souls of six other Great Evils. Doing so must have calbirated a part of their personas into him as well. One of this was arrogance, being Azmodan's biggest characteristic. My guess is that because of this influence, Diablo overlooked the one possiblity of defeat and thus allowed the hero the opportunity of freeing Auriel.

So basically, Azmodan ruined Diablo's chances of suceeding.