If you wish for a more precise answer, I would recommend contacting an admin, however I believe shooting the murderer and killing everyone as the murderer both help you equally rank up. To be more specific, if you shoot the murderer before they kill anyone else (or the least amount of bystanders killed) and if you're the murderer in a game full of lots of people and you kill them all; that will help you rank up the most.
If that explanation wasn't clear, feel free to PM me directly