It's mental illness, unless you can be construed as "right-wing" according to the bizarre strawman understanding of the right that these types have in their heads.

As a bonus, if a mass killer's schizo ramblings can be tangentially related to anything vaguely "right-wing," they also automatically become white, even if they're half-flip like Elliot Rodgers or half black like Chris Mercer. And even if, frankly, their worldview is far more congruent with a leftist perspective than a conservative one.