I've played games before with really good systems like this. The best I've seen is a game called Conquer, which is a terrible game, but has a great PK system. Everyone starts out being a "white named" player. Whenever you kill a "white named" player, you get a PK point. For a duration after attacking or killing a white named player you get "red named" which means people can kill you and not gain pk points. If you get more than 5 pk points you remain red named until you drop below 5 points (it took 30 minutes of in game time to lose 1 pk point.) If you get more than 10 pk points you get black named. Black named players don't get you pk points AND there is a bounty on their head (in that game it was their equipment, but since that already happens maybe they should just take a big penalty on their next lives for a duration or drop a trophy item or something?) At any rate, if this system was in place, you'd know not to trust red or black named players.