Roger Crockett was a homeless man who was killed by a female Jersey Devil in order to protect her offspring.
Background and Leading Mulder and Scully to the Jersey Devil
According from another homeless man who knew Crockett, Crockett and the other homeless citizens knew about the Jersey Devil and feared it as sometimes a female Jersey Devil goes into the homeless area of Atlantic City and searches through garbage cans for food. Crockett probably followed the Jersey Devil into the woods to have a better look at creature. But as the female creature saw Crockett, she assumed Crockett was a threat and killed him thinking Crockett wanted to cause harm to herself and her offspring. Crockett's arm and shoulder was eaten off by the female Jersey Devil and probably her offspring.
Because of Crockett's death, FBI Special Agents Mulder and Scully were sent to Atlantic City to investigate his demise and ultimately lead Mulder to discover a female Jersey Devil. (TXF: "The Jersey Devil")