...No, not the other guy either.
Let's take a break from all the pressing issues and talk about Tathamet.
Whom, if you've listened to the in-game conversations with Tyrael, was The Original Evil -and, what a surprise, also a dragon.
Not slain, but torn to pieces in a cataclysmic battle by the Original Goodguy, Anu, who in turn was also torn asunder by Tathamet.
Mutual dismemberment notwithstanding, neither of them quite met their end;
Parts of Anu formed the heavens, and the angels within. Also the worldstone, which, later would be used to create the realm of Sanctuary, the very plane of existence we're trying so hard to preserve in the game.
At the same time, Tathamet's body formed the burning hells, and his seven heads, each representing a seperate sin, became the seven evils.
Tyrael knew this all along. And that's the weird part.
How could he have agreed to collect the souls of the prime evils in one stone, risking to unite them?
Better yet, from three failures (one in each previous Diablo game, plus the first one already started with Diablo dead, and about to reincarnate.), shouldn't it have been obvious that such creatures cannot be killed? That they'll just re-form over time?
Instead, he went along with the black soulstone plan. Brilliant. Of course Diablo was masterminding it, and he took control of the power inside.
This time, they were slain as one. All seven of them disrupted, their essences swirling in the same stone, inevitably about to fall on some luckless speck of dirt (my guess is still the pony level).
So what's to stop Tathamet from being re-born? All the ingredients are in place, and this time, there's no one to take control over it.
Truly a brilliant plan. And this guy proclaimed himself the new Aspect of Wisdom.
Mark my words, if this game gets to see an expansion, this is what it's going to be about.
After all, that's pretty much the only way you could have a more powerful endboss than Optimu... err, Diablo Prime.