well, I've read that evil npc will leave you if you have too much much reputation (above 18). I have 20. So how can I lower my reputation ?

I know that leaving Viconia and take her again will lower my reputation to 18, but I read that when you leave a char, you totally stop the Romance you have with.

Any suggestion ?

I have read that if you go into a building and hit/kill a commoner, it will lower your reputation by one. Don't do this outside, however, as the locals will all turn against you.

ah, the problem of reputation. i suggest you do a search for that here. there was a very recent post just like this not a week ago...

try stealing from an indoor merchant you'll never return to. also attacking a commoner with fists (don't kill unless you want to lose 8-10 rep points).