Murder is a crime that Marston can commit. It involves directly killing an NPC (if you do not kill them it counts as assault). You get a bigger bounty for killing a law officer than if you kill a regular NPC. If you have high enough Fame and Honor NPC's will only report this to the law on the first murder committed. Committing murder can cause a bounty to be put on Marston's head, with the starting price going as high as $50 depending on the person killed.