I remember reading years ago that serial killers start out torturing and killing animals.
That seems to be the consensus, yet,somewhat more obscure/less defined, [Ted Bundy, David Berkowitz and Jeffrey Dahmer, sic] when sexual predation is more black and white.
[A study done by North Eastern University and the Massachusetts SPCA found that people who abuse animals are five times more likely to abuse humans than people who do not.]