I too have watched Not Just Bikes.I think the city planning in America makes people unhappy.
But really the US school shooter problem started because of the media sensationalism over Columbine. You can tell because it went from a freak event to a regular occurrence and these people worship Klebold and Harris as heroes. Rather than off yourself normally, you might as well do something that people will remember for the next 10-100 years.
These people don't randomly pick up a gun one day and start blasting. They plan meticulously for the camera. It becomes an all consuming hobby, the last semblance of meaning in a meaningless existence, culminating in a magnum opus of an ending. And as they find entertainment writing the script, we are entertained by watching the play unfold and then getting the behind the scenes. No matter how terrible it is you and I love this stuff because it's novel. We would rather feel any feeling other than boredom, as long as it doesn't personally affect us.