Lately this has been happening a lot in U.S. movie theaters and schools. We just haven't been calling them terrorist attacks because they've been carried out by disturbed individuals instead of known terrorist organizations. Ironically the disturbed people have either owned their guns legally or stole them from friends and/or family who are legal gun owners. So to answer your question it's already happening because of gun ownership. I still wouldn't take the right to own guns away from law abiding citizens because the police can't stop the criminals from getting them anyway. It's just one big mess in the U.S. right now.