The sort of people who would normally commit mass shootings in OTL are likely the people that are handed guns and told to go round up their neighbors, or make some of them disappear. Basically, they have an outlet and job satisfaction.
In some cases this is correct. I surmise that a person like
Patrick Sherril,
James Oliver Huberty,
Joseph Wesbecker, or
George Hennard could find work in being a member of either the Liberty Legions/Battalions*, the Holy Battalions, or police/army/National Guard, since those occupations come with the potential to cause extreme violence in a "socially acceptable" manner, but do not assume that this would be true of every (or even a majority of) would-be mass shooter(s). The act of mass murder is oftentimes coupled with the act of suicide, if not in practice then in spirit. Mass murder is usually the result of someone who feels deeply inadequate about themselves combined with an intense rage against the wider world and their fellow human beings. Essentially, they want to kill themselves but feel like they should ruin as many lives as possible before clocking out for good.
With that being said, the motives of mass murderers can be incredibly varied, and might not be connected to reality. For example,
Edward Allaway had delusional beliefs about people putting his wife into porn movies.
Richard Farley stalked a co-worker for years and lashed out violently when she got a restraining order against him. ITTL, with the total breakdown of the mental health system, not to mention the incredible amounts of stress the kind of poverty ridden shithole Rumsfeld's America and the CV's theocracy have created, you could easily end up with many more mass murderers than we've seen IOTL.
*I've seen them alternatively referred to as Legions and Battalions, so I wasn't sure which version to use.