And then you are the best assassin ever.
Not much of a curse in a killing intensive game.
Anyway...curses:
-Your eyes constantly bleed, making your sight really bad (auto fail spot checks, all enemies gain concealment 20%)
-You get an electrical shock every time you touch a metal. (say 1d6?)
-Nobody believes a word you say, ever. (auto fail intimidate, bluff and diplomacy.)
-Every time you kill you have a chance to die. includes unintelligent creatures, even pests. (1% so you won't die too fast.)
That's all I got for now.