From Pangaea Wiki
Murderers are players that have killed other players. Since they are criminals, they can be attacked without penalty and guards will be hostile towards them unless they belong to the same religion.
Becoming a Murderer
You can become a murderer when enough people report you for killing their character, since killing other players is against the rules unless it is done in a Role Play (need to provide evidence of the roleplay with a screenshot of the journal).