Well, since there are white men and legal guns in canada, but not as much mass shooters.

Also, in Europe, you have white men, some with guns (for hunting and far more regulated) and not as much mass shootings.

That points to something cultural, not mental health issues (since propensity must be similar in canada and Europe). But maybe opportunity has also something to do.

Maybe white men in US are in a position where they can become mass shooters easily and other countries have better mechanism to stop potential mass murderers. Not just police, but people who would talk down the mass murderer before he has the idea.

But it is just guessing.