I have a few questions to ask:
1. Is Diablo truly dead?
-The angels dumped his/her carcass outside the tower, and his/her body disintegrates as it falls to the ground. His/her soul, which I assume are those black-ish orbs because Belial and Azmodan's souls are just like that, is not found. So it's either Diablo is truly dead or simply contained in the Black Soulstone.
2. Are the other Evil's souls still in the Black Soulstone or were they destroyed when Diablo died?
-It was not actually explained to me the specifics on how Diablo became the Prime Evil. All that was given to us was the ritual and Adria firing a red beam at Leah, in which her soul was replaced by Diablo's. I wonder if the other Evil's souls were meshed with Diablo's, making him the Prime Evil, or if they were still imprisoned in the Black Soulstone, and as a result, Diablo having no political enemies trying to stop him, so he simply proclaimed him/herself supreme over all of Hell. And that brings to my third, and hopefully last, question....
3. Why did we simply threw away Diablo, and with that, the Black Soulstone, when Diablo was defeated for the third time
-No explanation needed. You just have to watch the final cinematic once more to understand. Oversight, maybe, but I really doubt that since this is Blizzard we're talking about.
Thanks in advance.