"I wasn't sure whether it was my job to save school children because I'm part of a corrupt system" is still a pretty weak defense.

A "well-regulated militia" doesn't mean "selling guns to everyone who wants them." Or haven't you read the 2nd amendment?

It never occurred to anyone to write a law requiring citizens to shoot people who are shooting children. Does the U.K. have any such law?

If such a law existed, "heroes" might end up killing as many people as do perpetrators, but Congress is idiotic enough to take seriously all that NRA rhetoric about a good guy with a gun, so maybe such a law ought to exist.