1. The more places we allow guns, the more places we're going to see shootings - the kind that are done by law abiding gun owners who suddenly stop being law abiding. These would far outweigh the relatively rare mass shootings we now have at gun-free locations.
2. Contrary to the popular pro-gun lie, mass shooters do not seek out gun-free zones for the target-rich environment. Almost every single one of these guys simply goes to the place of their grievance.