That's completely different, though. You simply can't generalize from that incident that sane people can safely carry guns in crowded civilian areas to return fire on crazy people. A clock tower's a far cry from a movie theater, a library, or a lunchroom.
Think about the sort of situation holing up in a clock tower creates. He was barricaded in a small, unique location almost completely separated from the rest of the campus. Everyone could instantly understand exactly where he was once you told them, even if you were all screaming and freaked out. Anyone with a gun could shoot that clock tower and have virtually no chance of hitting the wrong person. Of course it was safe and easy to fire back at him.
That's simply not how "active shooter" situations play out these days, though. All the stories I recall lately were people who pulled guns in crowded places and simply shot at everyone fleeing and/or roamed around shooting more people. Having a bunch of random frightened people pulling guns could make everything much worse just as easily as it could make it better.
Unless the shooter(s) simply continues walking around slaughtering people until taken down, tracking them down in moments of quiet would be an absolute nightmare if everyone is holding guns. Law enforcement are going into a fucking minefield if there's dozens of guns visible in various locations and no more than a handful are actually criminals. If armed private individuals start looking for the shooters it gets even worse; someone is going to shoot someone else with a gun, and the odds would be against them shooting the right one.
Arguments about letting random people carry guns anywhere and everywhere also completely ignore the fact that practically no one in the US possesses the necessary skill for ferreting out a shooter or firing back with lots of innocent people around them. The police aren't even trained nearly enough in basic marksmanship and urban combat in most municipalities; random folks with a concealed carry permit can be even less skilled than the police just as easily as they might be more skilled.
Bottom line: if the people who think they should get to carry guns in schools, hospitals, courtrooms, or wherever they're arguing about today want to pass a thorough Urban Combat course and obtain NRA Expert handgun badges then awesome; I'll feel safer with them around. Until then I simply cannot conceive of them going more good than harm.