!King_Amazon!'s Diablo 3 Plot Prediction!

I disagree! ...

Tyrael would unlikely willingly aid the Prime Evils (Three Brothers) as it was he who stopped them long ago during the Sin War (and it was probably he who banished them to their soulstone-y prisons.) When Inarius (generic fallen archangel) created Sanctuary, he, some angels and some demons created the Nephalem - who later became the mortals of the Diablo-world. If we can take the books as canon, it seems that Tyrael has been the one angel that really liked mankind and voted to keep them (I assume the High Heavens wanted to get rid of Sanctuary because of it being created by fallen angels and (risen?) demons.) Tyrael seems to be the one really good guy in all of Diablo, he has aided mankind more frequently than other agents of the High Heavens and as such is considered a "rogue" of sorts.

If all of his benign protector of the human race-thing is a front to keep his true evil scheme of letting the Prime Evils roam the world freely, it would probably have been easier for him to simply not form the Horadrim during the Sin War.

Tyrael might however have been corrupted in some way, like Izual or random and is no longer happy go lucky Angelman. If so, yes - it could be likely that he aid the Prime Evils in some way.

As for the prophecy, I think it only refers to the expansion pack. All of it seems to refer to in-game events:

And a child will cradle Terror in his breast as the heart of man falls under the shadow.
Events of the first game, son of king become host of Terror. Epic hero thrusts soulstone into skull, for unknown reasons.
A Wanderer will pass through the ancient lands trailing chaos in his wake.
The Warrior from Diablo 1 becomes the Dark Wanderer and starts Diablo 2.
The Three Brothers will be reunited as the mortal world trembles before their might.
The Wanderer collects Baal's soulstone and goes to meet his brother Meph.
And so it was foretold that the Three, once reunited, would be shattered again—
Epic Hero #2 kills two of them and one runs away, thus they are shattered.
Their defeat would be illusory—that the final gambit had yet to be played..."
They did not really lose as Baal is still alive, he wants to gambit with the Worldstone.

"And now at last the storm surges forth from the southlands, and the hand of Destruction
Baal lays siege upon the the lands of generic northern guys.
Reaches out to undo the workings of the Ancients. The tides of Hell surge—ready to smash
Baal wants to open the gates for the really huge amount of demons that want to smash.
Down upon the shores of the mortal world — to drown the guilty and the innocent alike.
This will not cause happy times.

A very easy explanation would be that destroying the soulstone (their prison) would merely let the soul/spirit/essence be reborn in Hell, thus the heroes merely freed the Prime Evils from their prisons. This is however not very optimum for the Prime Evils as the Burning Hells are ruled by the Lesser Evils Belial and Azmodan. So they might need to crack some skulls before they can rise again.