When we first launched the NPC Server I removed the whole AP system, including sainthood and health restrictions. Instead I put in a sort of PK 'rating' which was a score between 0 and 100 based on how good you were at PKing. Players who killed a lot without dying would have scores closer to 0 (realistically anything below about 30 meant a strong attacker). PK 'rating' was mapped against the AP colours, so that PKers had black nicks, players who died a lot had gold nicks, and players who avoided fighting had white nicks (Like 0AP, 100AP and 50AP respectively), with the whole scaling from 0 to 100 used in between. The idea of the system was simply to just let people fight without restrictions, with the rating used as an indictor to show what a players preference was.
I've always felt that the only reason people like being restricted to 3 or 5 hearts is because it allows them to get kills (and/or deaths) more quickly, ultimately inflating their stats. The fact that you die more often took away some of the enjoyment from PKing for me. I can see the arguement for not leting people who want to PK heal, but restricting their health just didn't work for me.
Eventually it was the players who voted to bring back the AP system, and that is why we have it. I personally wanted to do something similar to the 'Player Killer' tags mentioned by Stryker. I always thought these worked better than AP. From what I remember there was no sliding scale; either you were a Player Killer or not, and after 10 minutes without killing anyone the tag would get removed. There was no such thing as sainthood either, so people who did not want to fight were forced to run away. I've checked and it is possible for us to display '(Player Killer)' without interfering with any of the guild stuff.
I guess changing how the who PK/sainthood system works is something we can look at.