ANGEL OF DARKNESS: The True Story of Randy Kraft and the Most Heinous Murder Spree of the Century by Dennis McDougal (Warner Books: $19.95; 330 pp.). On May 14, 1983, while cruising Interstate 5, two highway patrolmen pulled over a man in Mission Viejo on suspicion of drunk driving. It was a routine stop--until a patrolman discovered that the Toyota's front-seat passenger was dead, strangled with his own belt. So ended, by pure chance, the horrific crime spree of computer programmer Randy Kraft. It's a story notable largely for the fact that the spree lasted more than 12 years; because Kraft usually killed marginal young men--drifters, gay hustlers, AWOL and near-AWOL Marines--there was little political pressure to catch him. Dennis McDougal, an investigative reporter for the Los Angeles Times, has gathered together all the unpleasant facts, but the portrait of Randy Kraft that emerges doesn't tell us much about his motivations, other than to suggest links to Kraft's homosexuality and love of bridge.