It depends a lot on the server like for example Existence Ragnarok Online uses its own unique PvP system which track's player's kills and gives them an aura which other players can see to warn them the person is dangerous and has been killing too much. As the points scale the aura color changes:
Red Aura 100-199
Blue Aura 200-299
Black Aura 300+
When a player hits 300+ and is killed by another player, it is possible that they could drop some items and they are sent directly to jail where they have to wait for points to drop, or bribe the jailer to let them out early. Player's who have killed too many players can hide in the market or paradise island and wait for their points to drop.
Players who pk others get experience and part of another players money that they are carrying people that kill others who have been killing too much gain a higher rate of experience and money.
Money which is given to the person going around pking others is automatically set into a bounty system over that players name. In the future others will be able to take a quest to kill that player and can claim the reward for doing so.
As you can see this is a specific type of PvP. Not your average PvP you would find in retail, nor most private servers because it is highly customized.