Well, the town already has the guards as their attempt to defer unnecessary assaults, and when you get strong enough to kill them, they are the only real big source of XP players may not be inclined to kill anyway. I'm not sure if the extra XP from kills is even a big enough deal to worry about, other than in the case of slaying every neutral guard. I imagine the XP requirements to level will go up enough each level that having several more low-level kills won't make a big difference to a character after a couple of levels.
