PVP is completely immoral from a roleplay perspective, however, if you must PVP, you will be limited to attacking players within four levels of your own level.
Any player who casts a beneficial spell on another player who is more than four levels above their level will be flagged Free-For-All (FFA) for five minutes. While flagged FFA, a player of any level is able to attack this player.
Some competitive zones are set permanently FFA and allow any player to be attacked no matter the level. When loading in a FFA zone, players will receive a message informing them of the danger.