He went crazy one night and killed six people in two different homes. They call him the "Vampire Killer" because he bites his victims on the neck during or after killing them. Last anyone heard, he went east. Bob the Bounty Hunter in Hoover Dam has a bounty out on Diablo's head.
When he dies he drops a severed head (presumably Bob's). Bob knows a guy who was going to look for Diablo out in Dog Town, but he never came back... El Diablo got him too.