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