Likely many things, from structural issues to non-structural ones.

And people do horrific acts against others.

Even going beyond individuals in need of mental healthcare access having easier access to guns... We see the USAmerican regime kill people, with their people's dictator making threats that they will 'end a whole civilisation'

I think they could reduce the instances of shootings by ensuring that people are taught about the gravity of such weapons, if they are in places that celebrate them