From Diablo Wiki
He is one of the most interesting characters in the Diablo universe, if nothing else for his appearance. His allegiance is also hard to figure out, could he be a wolf in sheep's clothing?
What we DO know is that he eventually saved Sanctuary from the Angiris Council, created the Horadrim to help us fight the Prime Evils and he has stood by humanity when we have been besieged by the forces of Hell.
Read more about this famous Archangel, and his story on the Tyrael article.