Okay I am going to jump in on this discussion
Only referring to Spree/Mass Killings like Columbine/Virginia Tech and so on ... "School Shootings" have been generally happening since the 1850's in the USA. Most of these events are a personal slight/revenge type of attack involving someone the shooter knows - and, "most" of the shootings before the mid 20th century involved deaths/injured in small numbers.
It seems since the late 60's to mid 70's, we get casualties in bigger numbers. Then, by the 90's - 2000's school shootings definitely increase. Some of the biggest shootings have happened within the last 20-40 years.
It seems like the point is not just to kill the person that wronged the shooter. Now, most school shootings are planned with greater carnage in mind. Whether that involves molotov cocktails, pipe bombs/explosives, using fires at diversion sites before the shooting to throw off first responders, being loaded with hundreds of rounds at the scene, and so on. There is more plan and detail involved in these shootings generally.
The media has definitely played a big part in this in the last 40+ years as well. The media attention and "to never be forgotten" play key roles in the shooter's plans. The "shock and awe" that results is very much what the shooter has planned to happen. Has the world become more violent ? Or, are people just reacting/striking back more violently ?
Nature or Nurture ? I believe it is both. We, as people, are affected by our environment/society. There is something called "diathesis-stress model" - it is a concept that shows how we both can share a similar environment yet our interpretation of it can be different. Some people may view certain experiences as positive or meaningful whereas others may view those same experiences as negative or destructive.
Obviously, school shooters were not BORN school shooters. People are shaped/affected by experiences in their environment/family/society at a very early age. I believe there are certain times in our lives where we are vulnerable as well. Shifting from home life to school life, moving from one place to another - whether a few times or often, puberty, graduating high school, leaving home - all these are critical times in people's lives. As well as marriage, divorce, death. Any of these factors can affect someone's decision to kill or not.
Most school shootings are a retaliatory action. It is a striking back against Society, Culture, People, Hypocrisy, Corruption, Injustice, etc. The sad thing is that now involves loss of more lives.
With all the school shootings that have happened over the last few decades, we have seen how anyone can become a shooter. You don't have to be bullied, or an outcast, or the quiet one in the corner or weird or whatever.
It is a sad fact that spree killings seem to be happening more often and yes, I do believe, part of the problem is with society. Discrimination, injustice, suppression - can all affect people differently.