The problem with this reasoning is that we have a lot of data on the mass shootings. In nearly every single mass shooting (62 in 30 years), there has been zero evidence to support the notion that the killer chose the spot because it was a gun free zone. Instead, we have plenty of evidence to support the claim that the killers chose the spot because of previous connections (they attended the school or worked at that building or wanted to harm the types of individuals found there [minorities or abortion providers]).
Next, consider the fact that nearly every one of the mass shootings was perpetrated by someone who intended to commit suicide at the end. Safety is not a primary concern and I know, you can imagine the thought process wherein someone seeking their claim to fame will want to shoot up as many as possible before dying, but do you really believe that such a person - someone who wants to die after killing as many people as possible - will allow themselves to choose a new target because they are likely to kill 5 people before being confronted or killing 10-15 people before being confronted? Do we have any evidence of a single mass shooter deciding to switch targets because of that reasoning?
Finally, consider the fact that zero of the mass shootings were stopped by individual armed civilians. And consider the problems posed by allowing someone who is not subjected to the rigorous training that police officers, swat, etc. are subjected to in order to know how to deal with a mass shooting situation. And whereas we have zero evidence of a mass shooting being stopped by an armed civilian, we do have direct evidence of an armed civilian confronting the assailant and that confrontation resulted in severe injuries to the civilian in Tacoma, Washington (2005) and Tyler, Texas (2005). Are they commendable for trying? Sure. Did it work? No.
How about this, everyone needs to discussing the topic not each other. That also means attempting to bait people into discussing yourself. Stick to discussing the topic
