Sad this young man made several fatal mistakes. Bringing the gun to school in the first place and by pointing it at a police officer with threats to kill everyone, he put himself in jeaprody. Under the circumstances, (In my opinion) he gave the officers no other alternatives. I too, have been trained to do what the officers did.
Actually, the first mistake was buying a pellet gun that looked like a real weapon under the assumption that it would scare someone.