Class/race/cabal combo.
Style: offensive/defensive.
Player mass killing skill. Don't confuse with true power, exploration, RP skills.
Triggers/link.
Not to be overpowered, else people will avoid you.
Often have perma in wide sense. People to quickly rank with, people to gear with. Thus it takes much less time to setup the character and leaves more time for fights.
Well timed loging/logout policy: no ragers without head, logout against strong or boring pk range, login against sheep pk range and make a run around ranking areas/cabals.
Low level PKillers may rack a number of kills. And while skill matters in this case, the most important fact is that they are usually greatly over-geared for their level. You can kill a lot of people if you use hero set at 20+ and probably less people if you walk in fine leather.