Ricky and Dena

If her case goes to trial, Dena Riley likely would use her drug addiction as an excuse. She might also claim that Davis had control over her — physically and psychologically.

But what excuse can Ricky Davis claim?

He seems to be a classic sexual psychopath — a person who uses sex to dominate another human being, turning them into possessions. And his videotaping of the assaults placed himself in a class with the infamous Canadian sex killers Paul Bernardo and Karla Homolka.

"Psychopaths have no empathy," said Dr. Seifert. "They get a thrill out of hurting people. That is what sets them apart from run-of-the-mill sociopaths, who can feel bad about hurting people."

And when a lack of empathy is combined with a violent sexual compulsion, you get Dennis Rader's "factor x."

"I think he (Rader) was observing that he had something inside of his head that drove him to do awful things, and he knew there was no way he could stop except for being caught, killed or imprisoned for the rest of his life," Dr. Seifert said. "He knew he had this drive to kill and hurt, and he knew that it was something that the rest of the world didn't have."

Joel Rifkin, the serial killer nurse whose crimes were revealed in the early 1990s, told shrinks that he could not stop strangling women — even though he knew it was wrong — because the act of murder gave him an incomparable sexual euphoria.

Dr. Seifert said Ricky Davis was another member of the same club — the killing kind.

Is "factor x" a force of nature, nurture or both?

Dr. Seifert has noted that a chronically violent person "is a matrix of complex interactive systems."

Some sex offenders have a genetic history of deviance. But environmental factors can play a role in how such a predisposition plays out in life.

Some learn to suppress deviant urges. But this may prove impossible for others — particularly those with a childhood trauma or detachment disorder that inhibit their emotional development.

And so it was Ricky Davis.

"He was in prison 18 years for a sex offense," Dr. Seifert noted, "and it did nothing to keep him from reoffending."