EDIT: It seems that some people are totally missing the deliberate sarcasm in what Michael Moore is saying here. The point is that Adam Lanza’s mother WAS a gun owner. She had been shot by her son with her own guns.
If you are one to make an argument that more guns would have stopped the killer’s spree, please consider that fact.
But if someone is already willing to break a law and kill people, not being allowed to have a gun isn’t going to stop them. It will, however, save some people. If other teachers had had guns, then the killer would have been stopped sooner.