In Rosh Online, conflict and war plays an integral part of the game. All players level 36 and above may kill and be killed by other players. At level 36, the PK button on your player interface becomes usable. When the PK button is on, you are able to target other players for combat; turning it off will ensure that you don't accidentally enter combat when you don't wish to (while hunting, for instance). The PK button does NOT protect you from being PKed by other players.
Player-killing in Rosh Online is guided by the Moral Value system. If you kill players, your Moral Value Score (shown on your character bar) will decline. If you engage in PK a lot, your MV score might fall to a point where you start incurring penalties: town guards will attack you on sight, your access to shops is restricted, and you may drop items when killed and earn less experience. However, killing monsters and players of low Moral Value will increase your MV score and remove these penalties.