The woman responsible for the capture of War Machine says she's dying of cancer and has decided to pay the $20K reward forward.
Mary Casamento was staying at the same hotel where War Machine was hiding ... a fugitive from the law after allegedly brutally beating former GF Christy Mack.
Mary says she heard commotion and saw a woman run out of his room ... bloodied. She called cops who came and arrested him.
Mary scored a $20K reward -- $10K from Christy's family and an equal amount from "Pawn Stars" star Corey Harrison -- but she's not cashing in, because she already has enough to last her for the rest of her life. She's decided to give the dough to a Santa Clarita, CA battered women's shelter.