Again, I'll never own an AR-15, but you are in favor of banning all AR-15s in hopes that it might stop one incident and that is good enough to pass a law. First, if you ban AR-15s, that doesn't mean that the minority of people that would commit violent crimes with them can't get their hands on it and even if it is hypothetically more difficult, we don't know how difficult. Drugs are illegal and they aren't that hard to get. Also, when you start passing laws because it might save one life, then you go down a slippery slope. There are lots of things that could be banned if it could just save one life. Yet you wouldn't ban swimming pools or certain types of cars or any number of things that take more lives (intentionally and accidentally) than guns.
I'm not a gun advocate and I'll never own a gun, but I just think its bad public policy to ban something that has little to no impact just for the sake of saying you are "doing something." I favor any reasonable gun control that will be effective for stopping gun violence. If banning AR-15s meant no more mass murders, then I'd be all for it. But the problem is that banning AR-15s won't stop them from being accessible on the black market and there will always be other weapons that take the place of the AR-15s.
And even if AR-15s stopped the mass gun violence, you would move the markers. Let's say there is gun violence where someone shoots 3 people on campus instead of 15. Aren't those three lives just as important as the 15. So how do we stop those three people from being shot? What guns can we ban to stop that?