I've only played the Starter version of Diablo 3, but I'm pretty sure that Tyrael wasn't a fallen angel. Izual was a fallen angel, but he was killed during Act IV Quest 1 of Diablo 2.
Izual makes a come back at the end of the game and Tyrael isn't refered to as a fallen angel here. he is thought to be an meteor of some sorts at the beginning of the game. you find that out right after the end of the starter version.