How about this, Guards have a range with they can deteat crime as well as npc's, who start runing around shouting GUARDS!!. When someone is caught attacking, killing, robbing, etc. they are punished by fines. Lets face it no-one is going to care that much if they die because they can just come back. They will care if they lose all their gold and even their items to pay a hevey fine (and then killed). If they can't pay the fine they are killed and a bounty is place on their head. The bounty is equal to what they failed to pay. If a guard spots you, makes you pay your bounty. If you can't the guard kills you and it starts all over again. That will make people think twice before random killing players.