29 years old and still waiting for someone to give me a good reason why a civilian needs to own a gun.
Since you said a good reason and good would be totally subjective and based on your opinions (which are anti gun) I suspect you are waiting on the impossible.

Let's keep talking about the how and not the why. You could just as easily throw a homemade pipe bomb into a classroom.
I beg to differ. I have no idea how to make a pipe bomb and I've taken college science classes. But I figured out how to operate a gun at a range after only shooting a BB gun before. In order to build a pipe bomb, you presumably have to go online and post in forums which are monitored by the DHS. Then you have to proceed and buy stuff for pipe bombs. I'm sure you can make one with relatively most homemade stuff but it would still be difficult, the guarantee of success that you actually built correctly aren't 100% and perhaps most importantly, you have to plan it out. We don't know how this went down but some gun crimes seem to be spur of the moment and even massacres like this seem to be that people snap. Guns don't really give you a chance to sit back and think. Building a pipe bomb, you have to go through all the steps which would be a deterrent in itself. Sure it wouldn't stop people like Columbine who planned for it, but it would stop other senseless crimes which happen at a moment's notice.