Every single mass shooter in Canada that I can recall had a firearms license.  One even used a registered firearm when the LGR was alive.  In the US they have lots of gun control.  Some states have more than Canada does.  What they need is perhaps more enforcement but certainly more services for mental health.  But ultimately if no one reports someone who they believe is on the edge, then no law in the world is going to stop them from going on some sort of killing spree.