Khan made Ermac, and Ermac was sorta like a prisoner under Khan, until Kenshi freed him. Now Ermac can do whatever the hell he wants. So he picked the side of good. LOL I don't know the full story.

*Spoliers from DA*
In Kenshi's konquest mode from DA the story says that he ran into Ermac in outworld and freed them from Kahn's mind manipulation. Now they are free to choose their own destiny.

o didnt know kenshi freed him, thanks

I think it has something to do with Kenshi freeing him, therefore he's sworn some sort of vow to release the souls of vanquished warriors (or something like that, lol).
Edit: At least the "Kenshi freed him" part of right, lol.

ohhh ya! i remember reading that he wants to make up for his wrongs, thanks

thanks, i was wondering about this myself.

Ermac WAS evil, even without being Kahn's prisoner. According to MKT had he beaten Kahn and taken over he would have been a tyrant just like Kahn.
Now they've just suddenly decided to make him good out of no where. Whatever. It's not that big of a retcon.

Anyanka Wrote: Ermac WAS evil, even without being Kahn's prisoner. According to MKT had he beaten Kahn and taken over he would have been a tyrant just like Kahn. Now they've just suddenly decided to make him good out of no where. Whatever. It's not that big of a retcon.

Hey, They change everyone's stories here and there. Remember Raiden in MK1? He was an arrogant God with a lack of empathy for living things who ends up destroying the Earth in his ending. Yet in MK2, He somehow becomes the protector of Earth and is a wise, Benevolent diety.