PK's and PKK's for short is a thing in Asian MMO's and mostly used in Animes like Sword Art Online or even the .Hack franchise.
And is my World PvP dream to actually have this on World of Warcraft.
An actual system of "bad guys" (PK's) and "good guys" (PKK's)
Would you like to have this in WoW?
If you have seen Sword Art Online you know what im talking about
Here is a video from a old game of a victim to Player Killers, and then, 8 months after...
http://www.youtube.com/watch?v=wjM24kMEagg&t=13m21s
Do you like the concept?
Note: This concept is even better when there is "overpowereness" in gear but in WoW we have PvP scaling...so is not as much fun BUT i would play it anyway