Full auto is designed simply to be a "repeater". There is nothing in that design that facilitates "killing large amounts of people very quickly". Fact is, any intermediate or large automatic rifle will be prone to rideup, this makes them much less accurate to all but expert shooters, most criminals are far from expert shooters. Now, crew served weapons on stabilizers like tripods, mounts, vehicle mounted weapons will have much more control and could mow down crowds by the hundreds, however, as has been pointed out they are not quite as mobile as a carbine or a m240SAW. Anyone who would try to gun down a crowd with a mounted machine gun during a criminal activity would be a complete moron, it's the textbook definition of a sitting duck.
Some autos have been successful in massacres during the gangland days, but these were ambush style attacks and if you look at the ballistics patterns they were not exactly efficient. More rounds were expended per kill than regular revolver fire would have accomplished. Those kills sent a message, a very messy one, and the public perception got the wrong message as usual(specifically the Valentine's Day massacre).