Correlation doesn't equal causation. Nor is there any reason to think that shootings done with an AR-15 are more deadly than others.
The Virginia Tech massacre was one of deadliest mass shootings (33) and was done with with handguns. Eric Harris and Dylan Klebold had semi-autos at Columbine, but they also had bombs, handguns, shotguns, and an elevated position overlooking one of the primary exit routes from the school, so I don't think there is any reason to think that the bodycount that day was primarily a result of one of their weapons being a semi-auto rifle. The massacre in Charleston was done with a single handgun. Gabrielle Giffords was shot with a 9mm handgun. Holmes shot almost as many people with his shotgun and Glock 22 as he did his MP15 with the 100-round drum magazine.
People use AR-15's because they are available, not because they are especially deadly. Take away that option and someone hell bent on shooting people will simply choose another platform. You can get 33 round magazines for a Glock 17 and with two of those pistols clack off 66 rounds in seconds and reload at least one of them just as quickly as you can an AR15. Even a handful of the standard 17 rounds magazines is an awful lot rounds that can be fired quickly, and much more easily concealed than a rifle.
Evil exists and it doesn't care what type of weapon it uses. If you got rid of every single "assault rifle" in the US it wouldn't put a dent in the number of people killed in mass shootings (which, FWIW, is still an extremely low number).