as Justice falls upon the world of men.
Valor shall turn to Wrath -
and all Hope will be swallowed by Despair.
Death, at last, shall spread its wings over all -
as fate lies shattered forever.
As I write this, I cannot help but be reminded of the prophecy that "Wisdom shall be lost." I find it hard not to view Malthael's sudden departure as yet another sign of the End of Days. - Deckard Cain
Perhaps Malthael hates the idea of humanity regaining the power of the nephalem, so much so that when Tyreal destroys the worldstone that restrained the power of mankind he finally snaps (or perhaps had snapped long ago). He disappears and effectively becomes a fallen angel (thus is 'lost').
At this time, Tyreal is judged for his 'crime' and is banished, finally falling to Sanctuary, more specifically Tristam.
Imperius decides that with the worldstone destroyed, Sanctuary must go before their power reawakens.
Auriel is disturbed by this, and seeks Malthael to convince Imperius not to destroy Sanctuary, however in finding him she learns he's also lost it, in a way far worst than Imperius.
Malthael weakens her and helps Diablo (Terror causes Hope to fall to Despair) possess/fuse with her. This comes from the fact that Diablo seems to glow with inner power, and the shoulders closely resemble Auriel's own. The weird spike things coming out of Diablo's back could be the demonic version of the angelic wings.
Malthael suddenly shows up to Imperius and tells him that Diablo has claimed Auriel, which causes Imperius to just lose it completely and orders a full blown invasion of Santuary by the Heavenly Host.
This leaves only one option left for Sanctuary, either throw off the shackles of fate (no more being the pawns of Heaven/Hell) or get obliterated by heaven/hell/both. I'll be choosing option 1, thank you very much.
In the cinematic with Azmodan, it is possible he's referencing Malthael, as Malthael is probably the only being with the knowledge to create a soulstone capable of being implanted into an angel, Azmodan doesn't want Diablo back in any form, Malthael wants him back for the purposes of driving Imperius nuts.
And thus you have my two cents...for what its worth.