Mass shootings are just a symptom of a society addicted to instant gratification. The traditional serial killer kind of stopped being thing in the early 2000s. There hasn't really been a Gacy, Bundy, Kemper, Zodiac, etc. in the last 25 years whereas they were all over the place in the 60s, 70s, and 80s.
I know that there was a sexual component for most of those guys, but I would bet anything that most mass shooters would fuck their victims if they had the time before they get their heads blown off. I guess that's the price to pay for choosing to speedrun your psychopathy instead of spreading it out over a couple decades.