Lets make the Aura of Pain more like controls for the unholy weapon. Replace protection with Aura of Pain. Make it more powerful the more kills you have. Or make it a permanent spell that charge up everytime you kill someone, but if you die, you will loose all its strength and have to start over.

Let me explain a bit more how I'd like to see it work.
First off, it should be like protection until powered up. Lets have it based on kills, and not as a permanent spell. Make it start counting kills somewhere between 30-40(a balance issue, up to 30 the A-P shouldnt have too much trouble, they can be quite strong at low ranks). After a certain amount of kills, 10-20, it could start acting the way it was described. When the A-P takes damage, there is a chance the damage is reflected back to the one dealing the damage. (Note: The A-P would still take damage from the blow). Not all of it at first, just a small part, say 10% of the damage or so. The more kills, the more damage is reflected, and perhaps increase the chance that damage will be reflected(But it should only improve a little every 5 kills or so). We dont want to A-P to be able to reflect back all the damage all the time, do we?
Oh, another thing. The damage reflected should be determined by the damage the A-P takes and not the strength of the attack, or an A-P with A/B/S would be like a porcupine. Except it wouldnt help using a long weapon against him.

This way the A-P still have to kill people to grow strong, but will have it a bit easier building up his strength since it cant be lost by dying. Ofcourse, if you are having trouble killing people at first, this wont help much, but I never considered A-P to be a class for bad Pkers.
Ofcourse, this could make a well-played A-P a bit overpowered, so it would be up to the Imms to balance things out if the idea is to their liking.