Well saying that Tyrael is evil isn't prepostereous imo... There are several explanations for his actions. First of all, he does some very shady things such as destroying the worldstone etc. gossip with cain and the such. However, just because he helped you to defeat the prime evils does not necessarily make him good. Remember the adage "the enemy of my enemy is my friend." I'm not sure if my understanding is clear.. but apparently the worldstone will prevent outside forces such as the Prime Evils but also the Hosts of Heaven from entering the world of Sanctuary. Now that is all great and dandy.. but he just cut off a major source of good in the world and now the only POWERFUL outsider left on Sanctuary... would be Tyrael.
It could be some scheme perhaps?? Or maybe Tyrael is on the side of man and the other Angels will take offense to Tyrael intervening?