That assumes he saw a kid with a gun and did nothing while evaluating the situation and getting a 100% confirmation that he is indeed a crazy shooter. Which could really only happen when he saw this kid started shooting other kids. Then he calmly found a location where he would not accidentally hit any bystanders and without missing, terminated the shooter. If that sounds improbable for a real life situation, that’s why OP expressed his doubts.

There’s exactly 0 reasons not to use nitrogen for death penalty. Guaranteed death every time.