My Theory:
There's a whole generation of kids out there, many of whom are pampered, narcissistic and too thin skinned to deal with the realities of life. We live in a time of instant gratification where Microwave ovens are too slow and fast food takes far too long. Calling someone on the phone is too much of a commitment compared to sending a text or instant message. Behind the wheel of a speeding car, even a generally balanced person will fell justified when they become enraged by a driver who isn't going at least 10 mph over the speed limit.
Kids spend hour upon hour playing video games where one simply blasts everything out of their way. They've grown up with Social Media where one can anonymously weigh in with their opinion or spew hatred and unfriend or block anyone who disagrees. They've seen their parents, from the anonymity of their speeding car, become enraged and endanger themselves and everyone on the road, because someone dared to get in their way.
I fear that some of the kids today have become so isolated and detached from others that their view of reality is skewed. Those they view as enemies, aren't seen as real people. Every problem they encounter or difficulty they have with someone in the real world, feels like a life or death matter to them and they don't have the coping skills they need to work things out in real time. They deal with these things the only way they know how...swiftly and with the skills they learned in their imaginary world.
Combine all this with the availability of a gun and there you have it...