What about getting the bounty from the gold the murderer has, And if he/she doesn't have any, his money gets negative which means that when he gets money from any source (killing, trading or whatever), that money dissapears until he pays the bounty that was set on him/her.
If the character gets deleted, or the player uses another character, the bounty money that he has to pay is transfered to that new character. In the end that money would be paid from the PK account.
The bounty is set by the characters killed by that PK. Of course the character who puts a bounty on a killer also pays that bounty. The bounty can get bigger as more characters pay for it, but in the end it can't be exploited because the money received from the bounty is half of what it's paid in the game.
My english is really bad for explaining it so I would use an example:
PK has 100 gold in his bank
PK kills player 1
Player 1 puts a bounty of 100 gold (100 gold are withdrawn from his bankbox)
PK kills player 2
Player 2 also puts a bounty of 100 gold(again the gold is withdrawn from his bankbox)
The bounty is now 200 gold
Player 3 kills PK and cuts his head. Player 3 takes the head to a guard: he receives 200 gold (the 200 gold he receives is taken from the PK bank, the PK had just 100 so that gold is taken from him, and the next time he gets another 100 the gold is taken from him until he pays his own bounty)
if the player logs in with another character, the debt is passed to that character (and it goes from one char to another until he pays the debt)
If the PK character is deleted, the debt is passed to the next character the player logs in.
In the end if the bounty is 200 gold, the players who placed the bounty would have paid 200, and the pk another 200. So 400 gold are paid for the 200 bounty.
This way the PK can never get his own bounty (he would receive the same gold he loses), and it can't be exploited by a friend because they would loose money that way.
EDIT: well, after writting it I saw a problem. A PK could get a friend to kill him so that he gets rid of his bounty, so maybe instead of the PK paying the same amount, he should pay twice the amount.
So a 200 bounty would be 600 gold (200 by the people who placed the bounty, and 400 by the PK).
Also there should be a limit to the bounty...
And maybe instead of paying the debt with the money they get from monsters, trading all that, just receive 90% less from that money, so if they have a debt of 100 gold, and a monster drops 100 gold, they would receive just 10 gold, and their debt would be reduced to 10 gold (they paid 90g).