Well/ regarding tyrael turning evil i think it is a big possibility based on lore and stuff in D2. Blizzard obvioulsy know about all of this and may not make Tyrael evil because everyone has anticipated it. I also have to say something about Cain. In Hellfire the D1 expansions one of the books in the crypt talk about the soulstones. First of all they hadnt been made yet or you would be possessed,,but besides the point. It say that if the being encased were to break free it would go after someone weak such as a baby or an old man hmmm.... Cain seems pretty old and weak to me. Also note that Diablo most likely had no intention of possessing the hero in D1 because you were just defeated by him. Instead he took the advantage of the oppotunity and took you over as you attempted to encage his being inside you. Ok, back to Tyrael.
I for one believe Tyrael is evil but some people have not touched on many things. I do not think he was corrupted by shards of the worldstone but was in league with the 3 all along. Why did he destroy the worldstone? Well the worldstone protects the barrier between heaven, sanctuary, and hell. Now he supposedly destroyed it because Baal had corrupted it. If this is so then the worldstones power would dwindle slowly as the taint destroys the worldstone. INstead tyrael destroys it and instantly break the barrier. If Tyrael had told the heavans about the worldstone they could have had time to get ready for the war with Hell.
Cain has already stated that Tyrael has been working on his own agenda. He has most likely already been banished from heaven because they are not supposed to interfere with sanctuary which Tyrael has. So if he is int he meteor being thrown from heaven doesnt make any sense to me. If he is corrupted why would he go back to heaven..exactly.
Regarding him fighting Baal and Diablo vs. Tyrael in Tal Rasha's chamber he obviosly let them inprison him to make it look like he was good. perhaps the 3 were expecting some mortal to come and free tyrael, maybe not but he would play an important role in the master plan. Baal corrupts the worldstone and tyrael finshes it off so that it look like he had a legitimate reason to destroy it. With the worldstone destroyed the 3 can be reborn and enter Sanctuary with ease.
Now you wonder how they got in sanctuary in the first place... Well with the soulstones. Once the were imprisoned they could possess somone. It it was TYRAEL'S idea to make the soulstones. Perhaps he had this in mind all along. With this master scheme everything has come together perfectly and they will now take heaven on unsuspecting in an all out war.
That is my theory enjoy