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. Any player of any level is able to attack an FFA player, however, the FFA player is still bound by the +/- 4 level PVP range.
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.