Robert L. Spitz, the 44-year-old drifter who was awaiting trial for allegedly stabbing and bludgeoning his parents to death in Canoga Park, died of natural causes, according to an autopsy report. Spitz died Saturday morning in the jail ward at County-USC Medical Center, 11 hours after complaining of severe stomach pain. He refused doctors' requests to perform exploratory surgery to determine the cause of the pain. The autopsy revealed that Spitz died from peritonitis, inflammation of the abdominal wall, and had suffered a hemorrhage of his stomach lining, a coroner's spokesman said. Murder charges against Spitz were dismissed twice for lack of evidence, but a Superior Court judge reinstated them in November. He was being held without bail pending his trial this month.