I was asked this question by my anonymous source in the USA that I have called "The Chameleon" a convicted multiple killer - hence the anonymity. This is a very broad subject that can cover so many bases, that I am going to attempt to answer it with my own personal views. I will not resort to high falutin theories or psychobabble, I am merely a meat & potatoes guy that goes the simple route. Obviously, enforced sexual intercourse or molestation are sex crimes, but what other bases does it cover? Examples to start with? How about Jack the Ripper? He killed prostitutes, but apparently did nothing sexual to his victims. Were the killings of them his sexual thrill, the fulfilling of his fantasies? The pointing of fingers at Aaron Kosminski over the last couple of decades, and the fact that he had serious mental issues, could be construed as a factor, but to me, it points to him as simply being a deranged killer, not a sexual deviant. I do not regard the ripper murders as sex crimes. On the other hand, Peter Sutcliffe wore an upside down v-neck pullover over his lower body, with his genitals hanging free. Obviously, it was so he could masturbate over his victims after killing them. This makes these sex crimes.
But for many decades, lurid and sordid details were kept from public view, due to what would be termed, the "morals of the times." We had Gordon Cummins, the Blackout Ripper, during WW2, killing prostitutes and show girls. Then there was John Reginald Halliday Christie, the serial killer of Rillington Place, in Notting Hill, London. Christie, originally from Halifax, West Yorkshire, had endured sexual taunts in his early life, from girls. He was referred to as "Reggie no-dick!" But when he was killing, he at last, had power over them. There were claims that Christie masturbated over some of them, and maybe had sex with his victims whilst they were incapacitated. He was thought to have been a necrophiliac. The motive was obviously sexual, as he could do as he pleased with them, either dead or unconscious. He now had power over women, something he never had before.
The ongoing controversy over the Boston Strangler killings is like our own A6 murder -Hanratty - is one of those cases that will never go away. These are the cases that authorities want buried. Was Albert DeSalvo the killer? Despite what authorities come up with, there are too many odd occurrences that will not sway my opinion. Albert DeSalvo had a very abnormal sex drive, in that he needed sex five or six times a day. What he definitely did commit, where the "Measuring Man" crimes, in which he sweet talked women into letting him take their measurements, and even talked some into having sex with him. Then there was the "Green Man" rapes, in which women were raped, but on occasions, the assailant apologised to his victims. But as to the murders? The first number were elderly women, and violated with objects, with no rape. DeSalvo wanted sexual release. Where was his gratification with shoving objects into the place where he would have wanted his todger put? Then the victims became younger. There was staging of the crime scenes, for shock effect to the people who found the victims. Where was the kicks for DeSalvo in that? He simply wanted sex! He was fascinated by the murders, and was adept at breaking into apartments. How many crime scenes did he later break into, simply to have a look around. The shit only hit the fan when he was persuaded by another inmate - he was facing a good spell in jail - to admit he was the killer and he would go to a psychiatric hospital rather than a jail. But the fallout can be a legal minefield, as we do not have the right to an opinion like they do in the USA, so I err to the side of caution. The killer or killers were sexual sadists, DeSalvo was a sex addict, simple as that.