I always found it amusing, if slightly disappointing in my game that the reputation penalty for becoming the slayer seems to affect the PC in even the most remotest locations where no-one else other than your party would know that it happened. Thus how could it really be generalised to be affecting your reputation if your party is true to you and not spreading foul rumours of your deeds?
I guess its yet another programming limitation/ issue with the game.
I'm pleased to hear that there are mods for it for evil characters when the choice to become the slayer is often embraced rather than repressed! [img]smile.gif[/img]