Simply put, monkey see, monkey do. The more you see something the more likely it is to be imitated, even murder. The more attention it gets, the more normal it becomes. Mainstream media is consumed by this type of shock/tragedy reporting. When one of these events happens it gets hours and hours and hours of coverage. There are thousands of people sitting at home who can identify with someone with a grudge taking a few people out with a gun or bomb. You see something often enough in the media it becomes the norm. 20 years ago, you saw something on tv about a mass shooting and it was horrific but you thought it was the exception, not the norm. Sooner or later, it happens often enough, it's not unusual anymore. Guy takes out a couple of cops and it gets to be no big deal, you have to get into double figures before people really start paying attention now.
I'll give you an example from my own life. I have a close friend who lives in a small town of about 7,000. She's a mom with a 17 year old daughter. In Canada we don't publicize suicides but I think many people are aware that in recent years it's become more and more frequent that teenagers make youtube videos before they commit suicide. In the past 2 school years, her daughter has been to 4 funerals from her high school of kids that have committed suicide. Her own boyfriend committed suicide nearly a year ago after they broke up for the last time. It's become the norm in that small town amongst a group of kids, for their own reasons, to use suicide as the last option to relieve their pain. To the best of my friends recollection, she can't recall any teenager ever taking their own life in that town prior to the last two years.
I think it's because when you see others doing it, it all of a sudden becomes acceptable for you to consider it. Just like with mass shootings. See it often enough and some people think, "wow, I get it, I'd like to take a gun and pop half my office too". Monkey see, monkey do.
