Except killing people in town, how can I decrease my reputation?
For high reputation, my evil Korgan can't bear my team, and he runs away often. How can I do that?
I need my Korgan so much, for he is a master of both axe and hammer now.. hehe...

In later stage of the game, you can always change into slayer form, that will be -2 each time I think.
Otherwise, find a house in the city with couple of inhabitants, turn the party AI off and kill one of them. And if your reputation goes up too high again, go back to the same house and kill the other. You might want to back up the game before you do this though, incase the survivor leaves the house and went out onto the streets.
You can also lower reputation by taking the evil path for the kidnap/bury-alive quest.

He said something about not killing people. Stealing works just as well, but instead of lowering your rep by up to nine, it takes off two when you fail an attempt. Use a shop you never go to, like that tavern at the city gates.