In all the recent mass murders involving so-called "assault rifles", only the Las Vegas concert massacre relied on full-automatic fire (enabled by bumpstock). No doubt this was the result of the extreme ranges at which these killings occurred. There was no marksmanship involved, he just sprayed the crowd.

No, the full-auto selector is not the villain here, its just a red herring. I certainly agree full-auto fire capability should be illegal, but it is not what really defines an assault rifle employed in mass murder mode.
I think magazine capacity is the primary characteristic. No one would buy an AR-16 or an AK-47 for "hunting" if it only came with a 5 round magazine, like most hunting rifles.