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Korgan shouldn't be classified as evil
Or at least should be able to turn neutral. Why? Because of this:
That really doesn't come across as someone who is chaotic evil. Sure, when you meet him he's a blood thirsty killer, and a mercenary through and through. But Mazzy seems to be able to bring some of the good out in him, or at least make him try.
Maybe its just me, but I think that if you have Korgan and Mazzy in your party, Korgan should turn Chaotic Neutral. Someone who is still a berserker who is pretty darn likely to split you from throat to groin with his axe, but at the same time at least somewhat aware that there are those who deserve such a fate more than others.