That is true, but for a lot of serial killers, there is a sexual thrill, even if they do not do anything overtly sexual to their victims. They tend to be sadists, so they can get sexual satisfaction just from exercising that kind of power over an unwilling victim.
Masochist: “Hurt me!”
Sadist: “No!”
“get sexual satisfaction just from exercising that kind of power over an unwilling victim.”
And not getting caught and getting away with it.