disingenuous. he says for the serial killer, the fear is all part of the twisted fun. >> part of the enjoyment that serial killers have is not just manipulating the victims, and tying them up, but manipulating a community, and tying them up in a grip of terror. >> what's now known is that rader hadn't taken a life since 1991. his explanation to the psychologist about why he stopped at ten victims is once again chillingly matter of fact. >> why aren't there more victims? >> why aren't there more? it seemed like as i got older, i started making -- well, physically, i just wasn't up to it. somebody would have to be an older person. i just wouldn't be able to fight physically. i don't know. i think i was just starting to mellow out a little bit. >> fox believes it's not probably not coincidence that rader stopped committing murder