Just a side-thought to the discussion... I am annoyed by this unstoppable ability by the Evils to corrupt everything, even Heaven itself. I know that falling from grace is ever more compelling drama than redemption, but it is being overused as a plotdriver in this mythos. Really, nothing is immune from evil, not even the archangels - the incarnations of Light??
Shouldn't it be logical that it works the way around also, demons can be redeemed. If the Seven Evils are inprisoned and in the custody of Heaven - the center of all Good, shouldn't they get "corrupted"?? How could the demons not win the war if they can turn even the archangels and the Crystal spire. Just sit tight and wait for the light to become tainted..
Of course angels and good guys can become corrupted, but there should be some logical limits whitin the story!
I just realized: The Angiris Council could take the black soulstone, evacate the Evil, put all themselves in there and become the Prime Goodness... :D Preferbly by using the Nephalem as a mortal host!
I thought the flawed nature of the soulstones/BSS is the sole reason why the Evils can still corrupt their surroundings. I imagine if you stuck some Archangels in the soulstone and hid it in hell it would have a similar effect. Though I kind of view the angels and demons like fascism and communism. Both are ideologically on different extremes of the political spectrum, but are also very similar. The arrogance and hubris of the angels who deal strictly in absolutes would be just as harmful a corrupting influence on sanctuary as the corrupting influence of Diablo and his brothers.