What is it about America that brings out the lone psycho killer gunman? Europe has now and historically had fewer shootings AFAIK.
One big difference between Europe and USA is health care. Here nearly everyone gets checked out by a physician each year. They talk to their patients and can often tell if something is wrong psychologically. Then they can get a mental health check up, and it is cheap or free. People are less isolated in Europe too. If someone starts talking crazy, acquaintances will call them out on it. Not so in the USA. Nobody will say anything, and they will ostracize you.
In the USA , nobody goes to a doctor unless they have a broken arm or something, and they might avoid going their whole life if they can.
Maybe improving health care and social support is the solution. We all know gun laws aren't realistic