As I've said before, I believe mental health is sometimes a scapegoat. If it were mental health, this type of thing would be commonplace everywhere, as biologically originating mental health issues should distribute homogeneously across populations. For the same reason eating disorders are nearly exclusively an American issue, it's more a function of social constructs than mental health.
The Buddha taught that desire creates unhappiness. We're a culture that wants, wants, wants, because since we were young we were told we could have, have, have. Why have political scientists found that anti-immigrant rhetoric rises as economies fall? Because the more we don't get what we want, the more angry we get, and the more we look for someone to take that anger out on. Kids that shoot school bullies often feel deprived of the social status that others around them have. Timothy McVeigh was unhappy in life and chose to attribute his situation to the government. That guy who went into a gym and shot all of those women took classes on how to pick up women and still failed, and took it out on them. And by some accounts, the Orlando shooter targeted Puerto Riccan men because they had dismissed his advances so many times.
Unfulfilled wants -> anger -> violence. This is in sharp contrast to the Sandy Hook kid who suffered from a diagnosed mental illness, or William Hinkley, or others who had actual diseases you could point to.