The consequences come from your peers, not from invincible policeman guards.
As early as the 18th century in America, codified common law was not able to be sufficiently enforced by the government on the frontier. Does that mean murderers ran rampant at that time?
No. What happened was that if someone in your village or town was a real bad guy and broke a lot of society's mores and rules, the other members of the town would band together to either banish, kill or otherwise punish the bad guy. This is more or less how it works in Mortal.
Right now I live in Jungle Camp, which is a 'lawless' unguarded town. Does that mean that people run around killing one another for no reason?
Well... sometimes, but not usually. If a geared player starts to grief nakeds or people trying to craft, you can be damn sure that some band of heroes will eventually form to politely ask the griefer to please stop. This request most often comes at the end of a sword or arrow.
I agree that murderers, and especially certain types of murder (killing a naked, killing a person with no weapon, ect) should have some sort of significant brand associated with it, but I do not agree that being a murderer should affect the mechanics of the game. It should only affect how other players treat you, and it does that already.