Never trust anyone that goes to a concert alone. The last one shot up everyone he could.
I think a lot of people go to concerts alone, this is a bad generalization.
It was a joke. There is nothing to explain. The last shooter was a retard and missed everyone, but that's beside the point.
I think you are trying to play this off as a joke but you shouldn't. You are trivilizing the harm done, even if there wasn't physical harm done this time.