Britannia keeps score. Strike the wrong person and the town knows.
Grey: a criminal
Attack an innocent player or townsperson and your name goes grey for two minutes. Inside a guarded town the guards come for you.
Before a criminal act the game asks: "This is a criminal act. Art thou sure?" Dying ends a criminal flag, and the guards leave a risen body alone for a minute.
Red: a murderer
Each innocent you kill is a murder count. Five counts and your name goes red. Counts wear off slowly: a short-term count after 8 hours online, a long-term count after 40.
Healers and ankhs refuse to raise a murderer, and a murderer loses skill points each time they die.
Karma
Karma rises for healing others and defending a town, and falls hard with each murder. Shopkeepers notice: good karma takes a quarter off their prices, and a murderer pays half again as much, a Reviled twice as much.
Each murder also puts a bounty of 100 gold on your head, paid to whoever kills you.
Guards
Inside a guarded town the guards answer crimes at once, and strike any monster that walks in. Outside the town's edge you are on your own.
While you are Young (until your first clear of the Rotmire, or three hours of play) other players cannot attack you.