It's not that getting the max amount of XP is the most important thing, but increasing levels and getting stronger is an important part of the fun. With ever increasing amounts of XP being rewarded from quests and new enemies, and larger amounts of XP required for each level up, I just doubt the kind of min-maxing from farming every low-level NPC will ever be too much of an issue.
The reason it is currently an issue in the alpha is because if you kill someone from stealth that has nothing to do with combat normally you gain the same xp as from an actual battle. 450 xp is no small matter at level 2 and can easily make you gain levels if you kill all the npc's in the game which is what people are doing just so they can level saying that they are playing evil characters.
That is not evil that is metagaming because you know as a player they have no quests for you because it is a video game. Evil doesn't want to get caught anymore than good would want to get caught for stealing. Evil characters even in a PnP game don't kill everyone because the GM would do something to kill that character or punish them like having them arrested by high level knights or something because at that point they are just a disorganized serial killer and everyone knows who they are.
