Imperius is not dead.
In the Wrath video during the confrontation with Diablo, it is revealed that Imperius is not pure; that he has "sin" in him. The video is named after one of the deadly sins; Wrath. Therefore, I believe that Imperius is transitioning from the Archangel of Valor to the Archangel of Wrath.
I place my money on him being a boss in the expansion.