Minsc, though he rocks enough as it is, is a very depressing character. By that, I do not imply that he is boring; rather, I imply that his history is filled with angst, woe, and a good deal of cheerlessness. Personally, though we all love him dearly already, I believe he'd be a great NPC to corrupt. Not only would it be such a crazy swing of personality, turning him into a Barbarian wouldn't be too confusing. Then we finally get the NPC barbarian we've been waiting for, and can keep Minsc in our party all at the same time.
The one downside, of course, is that in order for Minsc to become purely evil, Boo would probably have to die. However, I could see Aerie dying (permanently) after Minsc adopts her as his witch causing him to shift to evil as well. However, I think he would be much more of a CN character after his corruption- not really evil, just crazy.