Not sure I understand your argument. Both statements are true. Had there been someone armed in the school, they might have stopped him, but if it was known that there was someone armed in the school, perhaps he wouldn't have gone there at all.
Banning guns doesn't prevent murder. Look at Ciudad Juarez in Mexico.
So yes, if everyone was packing, someone could still murder lots of people with a bomb vest or some other explosive device. In a crowd they may even get a few shots off before being brought down.
What you have done is stated our arguments, made an obvious statement, then provided no counterargument. That was a very productive post.