It seems that reputation = 17-18 is the highest rep that allows to keep evil members - it's true regarding Vic and Edwin, though I'm not sure about Korgan - usually I have him around long enough only to do his quest. When rep goes to 19 Edwin and Vic leave for good. The perfect way to keep reputation at needed level is Slayer change - you won't have to kill innocent people or do anything alike
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There is always a very hard way to keep evil friend in the party with high reputation - save/load, but it's so boring!
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Tanda,
with best regards from Guild of Assassins