Curious about a Curse

[Hell_Born]

I was just idly reading the FR novel "Shadowdale" when I noticed something. One of the main characters, a Fighter named Kelemvor, suffers from a curse and I began to wonder; how would you replicate it's effects using Ravenloft's game mechanics?

The curse originally decreed that the cursed would transform into a panther (in mind as well as in body) and would remain in that state until it killed a sentient being; it activated whenever the cursed tried to perform an act for any type of reward (it was an attempt to punish a greedy mercenary who left his sorceress companion to die out of his greed to secure some loot, even though he could have saved her with no problem).

However, the curse not only passed down to the original victim's descendents, it also mutated; now, the cursed transforms whenever they perform any act for another (acts for the purpose of defending their own life are permitted) without receiving a reward (either because they asked for none or because it was witheld for whatever reason).