John started out being just another character who went around killing Zombies, that gradually turned into a Player Killer.
He wanted something new, refreshing.
Unlike earlier he wanted to be on his own level, not on the "Humans" VS "Zombies" level, but rather something more like "Group VS everybody else", this might sound just like Humans VS Zombies, but it's slightly different in the meaning not only having to be careful about Zombies but also Humans that kill other Humans AKA Player Killers or Bounty Hunters.
John joined Red Rum and started killing other players indiscriminately, he supports HAT though, meaning he wont kill other Player Killers, not including Bounty Hunters, at least he'll try not to.