The manga kind of glossed over it during the PvP event but if I remember right, the PK RP players mostly PK with other PKers. It's just the assholes PKers who were trying to level up asap were the problems and the RP PKers did in fact step in to deal with the offenders. Of course sometimes PvE players will PvP with the RP PKers but it's more like a cautious respect and the RP PKers do prod the player to make sure they're ok with the PvP (sure they act like assholes about it but they're at least giving a way to accept or back out). The PKers keep the PKers in check.