>>Does the professor realize that there was nothing stopping the criminals from carrying guns on campus to begin with?
Remember that economics is a “science” based on assumptions and models that all start with “ceteris parabus” (with all other things being equal). In his mind, a legal gun on campus means that, ceteris parabus, more guns are present on campus and, ceteris parabus, the threat of gun violence increases.
I’m not defending him because he’s an idiot. But that’s how economists think the world operates: one change at a time.
If that’s how economists think; somebody should probably tell them that nothing in life is ever equal, even less so among “all other things”.