It seems we have had a marked upturn in the number of massacres in the last twenty years. They were virtually unknown until the early 90s. The perpetrators tend to be white males 17-30 years old.
What changed in the last twenty years? Violent video games that let you pretend to murder dozens of people. The same age group that buys these games are the ones doing the massacres
What changed was designs from gun makers that provided for more powerful, high capacity weapons. Video games didn't make the kill shot.
It's like saying "There was Ben Franklin with that kite and key, and then 150 years later, there were people stepping on the third rail and electrocuting themselves by taking toast from the toaster with a fork. What happened? Why pictures of lightening bolts! That's the reason!"
What changed between 1966 when Charles Whitman climbed the tower in Austin and the early 90s? The assault rife, the Glock, the Saturday night Special.