What happens when a burglar is shot by a home owner and there is a dispute on whether or not it was justified? Almost every single act of self defense would be heavily disputed. You would have "vigilantes" dispensing what they see as justice by trying to apprehend the home owner, who would then again be forced to defend themselves. You would need a higher entity of authority to arbitrate these disputes, possibly with lethal force, which would essentially be just another form of police.