We have already seen that crazy killers who can’t get guns will use Airplanes, Trucks and cars to kill people. Then there are bombs, poisons, knives and swords. People kill people, and they do so indiscriminately, with or without gun control.
Not all mass murderers are “White men” either: the 9/11 planes were hijacked by Saudis, who consider themselves people of colour. You make a good point though, that mass murderers do seem to be always Men: Male losers with psychological problems, angry loners with distorted world views, who often post hate messages on social media before carrying about their barbaric acts. The psychological profile of an ISIS terrorist and a serial killer are very similar.
Maybe we should put our efforts into better funding for mental health services rather than into gun control, which is constitutionally difficult, and unlikely to stop these losers murdering innocent people; they will just use different weapons.