In areas with strict gun control but access to a black market, mass killings still happen with firearms (see the November 2015 attacks in Paris or the 2017 Quebec City Mosque Attacks).

In areas with strict gun control but limited access to firearms, mass killings still happen with other modalities. A lot of things can effectively kill a bunch of human beings assembled in a restricted area: knives, machetes, bombs, trucks, etc..

The idea that restricting one modality of mass killing will prevent all mass killings is misguided. Would it reduce the frequency or the lethality ? That's debatable. Someone who has decided to pursue mass killing as systematically as the Las Vegas shooter will simply find another weapon to kill en masse.