These are people who want to hammer the rest of the world into a shape that pleases them and only them. Never mind what other people want. This world scares them and the only tool they have to navigate this world is violence.

Sadly, this is going to get worse before it gets better, as these people become even more radical and terrified. On a more positive note, it is also a sign of their imminent loss of power and relevance. Its saddening to know that at some point they will go too far, and there will be a backlash to whatever it is. Extreme violence always comes before their loss. Remember that the dogs and waterhoses didn't make their entrance onto the Civil Rights scene until white southerners began to realize they might actually lose the fight to keep Black southerners oppressed. At some point their opponents (that's us) will disregard their violence and keep moving forward anyway, because we have to if we want to survive.

Violence is the only tool such people (as the shooter) have to bring the world to heel, because they lack the imagination to do anything else. They've tried it all, lying, stealth, reality rejection, and none of it is working. None of that is changing the world the way they want it to be changed. All they have is killing, and that has never worked in the long term (at least not in the US), because they have always failed to take into account that the rest of us are fighting for our lives. They are only fighting to take them.