Based on what previous school shooters have said over the years, the reasoning is somewhat simple and obvious. They experience trauma at a particular time in their life, and once they've mentally broke, the most personally "relevant" target for their anger is a group of people similar to those who either caused the original trauma or were present at the time but "did nothing to help." It doesn't matter that they're not the exact same people, because it's very rarely about revenge against specific individuals, but instead a way to exert control over a situation or space that they associate with their earlier trauma.
If one in every thousand Americans has a unique and unexplained mental illness that compels them to violence, that would provide more than enough explanation for motives. It's even true for some famous shooters. [1],[2],[3]
The urge may exist in some teenagers across the world. But they don't have the mean to. You can solve this problem either by fixing the urge to kill or make it impossible.
Uvalde shooter locked himself in a classroom than proceeded to kill 18 ten-year olds and their two teachers. Why?