I think it's something along the lines of...

PKing by itself ISN'T a crime, because that's the whole basis of PvP. However, going out of your way to ambush people (e.g snipe them while they're unaware, rushing at them with your friends while they're alone) is looked down upon. That's why matches generally happen with equal numbers on both sides, and each side understanding that there will be a fight going on. Basically, you can kill each other without worries if you have mutual consent, but being a meanie-face and killing them while they're unaware? noplsSweeper-chankillthemall