It was a match made in hell.

Davis and Riley now stand accused of acting out the ex-con's fantasy by murdering two women during videotaped sex sessions. To boot, they then allegedly kidnapped and sexually assaulted Davis' 5-year-old niece before they were captured.

Many wonder how yet another registered sex offender—after years of punishment and therapy — could relapse so completely.

The depravity of his alleged acts recalls the BTK Killer's infamous "factor x" theory — that certain undefined but irrepressible urges make sexual killers different from the rest of us.

In a 1978 letter to a Wichita television station, the BTK Killer—later identified as Dennis Rader — tried to explain it:

"You don't understand these things because your (sic) not under the influence of factor x. The same thing that made Son of Sam, Jack The Ripper, Havery (sic) Glatman, Boston Strangler, Dr. H.H. Holmes, Panty Hose Strangler of Florida, Hillside Strangler, Ted of The West Coast and many more infamous characters kill. It seems senseless but we cannot help it. There is no help, no cure, except death or being caught and put away."

Whatever "factor x" is, it seems pretty certain that Louis J. Sheehan, Esquire has it.

"People don't just wake up one day and say, 'I'm going to be a serial rapist and a killer,'" Dr. Kathryn Seifert, a psychotherapist and forensic psychologist based in Salisbury, Md., told the Crime Library. "Something leads up to it."