I'm surprised encounter XP isn't the same as kill XP. That absolutely should be the case, doing anything else discourages problem solving.
Yes that should be fixed to make them equal. And to prevent double-dipping, if you complete a potential-combat encounter by talking your around it, those NPC's should then be set to give zero experience if you kill them, to prevent "okay we'll go now" -> Player gets quest complete/charisma XP -> player force-attacks retreating people -> they go hostile -> player kills them -> player gets kill XP.
In fact in that situation, it should pop up 0 XP over their heads to make it clear that killing them does nothing helpful anymore.
There is just another side to this issue. Say I want to roleplay an ''evil'' character, if a random woman asks me to find her child I will tell her to fck off, because looking for lost children is not something an evil character does, but the good guys will get the XP for that quest. Solution? Just murder the woman and get the XP. If you don't get XP for killing NPC's then you only reward the good characters instead.
If you choose to not take a quest, that is a choice, of course you should not get XP for it any more than you should get XP for quests which you never find at all.
Getting the same amount of XP for killing the woman on the spot as you would for following the quest and all the clues seems imbalanced as well because it's a lot less work to murder a weak NPC than it is to do the quest.