We are kidding ourselves if we think we can crawl inside their minds and try to figure out their thinking.
The world we live in has changed. When I was growing up in the 60's and 70's, people for the most part were pretty isolated. Even if you lived in a big city, your experiences with other people were generally limited to your family, friends, neighbors, people at school or people at work. I certainly wouldn't be communicating daily with Cornell hockey fans and BU hockey fans and Michigan Tech hockey fans, etc...
If you had crazy ideas or thoughts running through your head about killing and violence and what not, there is a pretty good chance that you did not know of anyone else who was experiencing similar thoughts. I think in some small way that had the effect of suppressing action on those thoughts.
Today, everyone throughout the world is connected to one another. If you have moonbat crazy ideas running through your head, it is extraordinarily easy to find others having similar thoughts. That connectivity provides support. It suggests to you that maybe you aren't crazy, that maybe the actions you are thinking about should be acted upon.
I don't know. Maybe it's complete bs, but I have a feeling that news coverage we see from these tragedies and the reinforcement the killers get from others they've managed to connect with is playing some role here.