AR-15 bullets travel almost three times the speed of a typical handgun. When a bullet from a hand gun enters the body, in travels in a straight line and either stops or goes all the way through. When a bullet from an AR-15 enters the body, it causes a significant cavitation effect because it moves so fast it sends shockwaves through the body. The bullets often will flip over upon impact, making the cavitation effect much worse. The result is that when someone is hit with an AR-15 bullet, there will at the very least be massive blood loss that will be fatal without immediate medical attention. Those who actually make it to the hospital have very poor survival odds because the trauma surgeons often find that organs that can usually be saved when hit with a bullet from a hand gun are just disintegrated when hit with a bullet from an AR-15.
AR-15s have hair triggers and are very light and stable. There is no weapon legally available that can better make an average person into a highly effect mass shooter than an AR-15.
The fact that there is an epidemic of gun violence does not mean that we should give the AR-15 a free pass. Banning the AR-15 should just be one part of comprehensive gun control. Mass shooting have had a disproportionate impact compared to generic gun violence. Mass shootings have caused nationwide psychological trauma. My 7 year old came home from school asking if I knew what day the bad guy would come to school so he could stay home. They had active shooter training that day. Security measures for special events are now ridiculously expensive and intrusive because of the threat of mass shootings.
What is really all boils down to is whether people's right to be free from gun violence is greater than the right of a very small group of anti-social gun nuts who have fetishized guns and fantasize about taking over the government because they cannot dump motor oil in the sewers and hate paying income taxes.