1. Male underachievement and lack of identity is happening elsewhere. Certainly in Canada. 2. Mass shootings are not happening everywhere
I think there is a much simpler explantion for male mass shooters: men commit vastly more crime. America has a cultural tendency for mass shooting crime, so it shouldn't be surprising men dominate that crime like they do the rest.
Maybe fixing male identity would do it, but that's far from a given, and given international experience it doesn't seem to be the direct cause. (Men committed most crime even when they were confident in their identity)
The more likely cause is the legal structure around gun ownership in the United States.