Q u o t e: Perhaps Tyrael is in fact evil?
In Diablo 2, Tyrael's help was extremely limited. You might think that an angel of his status would do more than just give out information.
Could it have been possible that it was in Tyrael's plan to let Baal and Diablo go after the cinematic at the end of Act 2 in Diablo 2? I mean it seems like Tyrael should have defeated both Diablo and Baal in Tal-Rasha's tomb, but he appeared to be overwhelmed by the two prime evils. Sure Diablo was in the form of Diablo's previous defeater and Baal was in possession of one of Sanctuary's most powerful mages, but neither Diablo nor Baal were in their true form and couldn't have been both more powerful than Tyrael.
Maybe Tyrael's ultimate plan is to defeat the three prime evils, forever banishing them, and then claim the world of Sanctuary for himself. The prime evils may have just been something to weaken Sanctuary's population for the benefit of Tyrael. However, this is just a theory. I could be completely wrong.
K, here's my take. About the standing aside part: remember act 4? He says that he has been directly forbidden to aid you.
On the note that two Prime Evils should not be able to defeat him: well first of all I don't think an archangel is equal to a Prime Evil in the first place, so two Prime Evils would defeat him even if they were weaker than normal(and of course their taken bodies do play in). Plus as you can see, Baal's binding power is just a bad match for Tyrael when it surprises him.
I really think Tyrael is good. If any angel is about to go evil it's Imperius, but he seems to be kept in check by the council. And to all those saying "Tyrael destroyed the Worldstone and is thus evil", well have you forgotten that a corrupted Worldstone(and Baal did corrupt it) would be worse?
I could also be completely wrong and Blizzard might have planned some relly weird plot twists, but I prefer to believe in Tyrael for the time being.
Warning! excessive use of Mreaper may result in: plague, parasites, psi-storms, 3rd degree burns, stab wounds, intergalactic war, ingrowing toenails, inertial dampener failure, and the apocalypse.