I don't think this is a problem. If Larian made the little quests you can do for people slightly more rewarding in EXP a 'good' helpful character could get enough XP to equal or beat characters who kill people willy nilly. And so both the willy nilly murderer and the quest giver will progress well, more loot for murderer, more quests for the good guy.
If somebody wants to go through the work of re-uniting two people, or completing a jerks quest and then attacks them. They SHOULD get more exp and the rest of us shouldn't care. In addition, if somebody wants to trade with a guy, then stab him in the stomach and take all of his inventory. He should be able to do that and the rest of us shouldn't care. It's a single player RPG game, and some roles that people will play should not be punished for being a power hungry loot grabbing maniac.
In short, the ONLY change to experience granted should be a small increase to errands and side quests to allow for a 'good' character to meet a higher xp milestone than someone who ignores quests and kills people.
BUT the guy who does the quests and kills the person afterwards should not be penalized. It's a valid character choice that is not power gaming or system abuse. Villians will often work with someone, maybe even 'help' them in some way to gain trust before taking what they want. Why should our PCs be any different.
