About tha cinematic in D2, tal rasha's tomb: Tyrael had a sword and was having trouble to fight Diablo in a human form, unarmed. then Marius released mephisto (or was it baal?) anyway, Tyrael was vastly overpowered. How did he survive? He surely must be a bad guy now, or maybe the whole thing was faked and he was already a bad guy!
That's not the way I remember it. I remember it as Tyrael beating Diablo down, about to deliever the final stroke, when Marius frees Tal Rasha (Baal). Then Tyrael is so busy admonishing marius Baal gets the jump on him.
How did he survive? Probably because the Diablo and Baal thought it more fitting to leave him bound in the same way Baal was until he finally passes away. Or to prevent him from interfering again. Angel and Demon can't be completely vanquished, when you slay them, they just reform in whatever pit (or heavenly cloud) spawned them. Maybe if they slew Tyrael then all of Heaven would have been forced to get involved. Or at the very least Tyrael could of just flown back to sanctuary and picked up from where he left off.
Tal Rasha and Mephisto were workinig together at the end to bring Diablo forth from the wanderer (that diablo model was the worst I've ever seen from Blizzard. It really did look like a 6 foot model of godzilla).
In the ACT 2 video, Baal was imprisoned and freed by Marius.