1) Timothy McVeigh killed 168 people, including 19 daycare-age children, and wounded over 600 more. He used a box van, fertilizer and racing fuel. We haven't banned box vans, fertilizer or racing fuel.
2) The 9/11 terrorists killed thousands using box cutters and airplanes. Neither has been banned.
3) If you want to talk about banning something, let's understand what we're talking about. Rifles with a burst- or full-auto setting are already heavily regulated. Semi-automatic rifles without such settings are all functionally similar: one bullet per trigger pull. The 1994 federal AWB restricted weapons based primarily on cosmetic appearances. Go run image searches for a Mini-14 Ranch Rifle and then for a Mini-14 Tactical. Those two rifles use the same cartridge, same magazine, are equal in accuracy. It doesn't make sense that only one would have been covered by the AWB. (Caution: The danger here is that you'll get a "then let's ban them both" response. You could also compare to something like the Winchester Model 100, which is also semi-auto, but chambered in a more powerful round (.308) than an AR-15 (.223).)
4) The Columbine shooting happened while the AWB was in full force and effect.
5) Columbine, Sandy Hook, Jonesboro, Virginia Tech, Appalachian Law School -- These all happened in Gun Free Zones. (I vote that we rename them Fish-In-A-Barrel Zones.)
6) One of the first documented assault weapons was a rock. Rocks have probably killed millions over the millenia. It's still legal to carry a rock everywhere you go, open or concealed.