Realistically, it's not a mental illness for many/most of them. It's an ideological and social problem first and foremost that may or may not attract some people with mental illnesses.

The assumption that it's always a mental illness is not only incredibly vague/ambiguous (excuse me, but since when is there a diagnosis for "active shooter" in the DSM or ICD?), it also stigmatizes people with mental illness by lumping them in with terrible people who may or may not even be mentally ill in any way.

People go out and kill people for reasons that have nothing to do with mental illness all the time. Gang fights. Ideological beliefs. Bigotry. None of those are mental illnesses.

I can understand going there if a shooter actually has a history of mental illness. But this whole trend of calling it "mental illness" as though it's the "compassionate" or "nuanced" way of approaching the topic is just plain bigotry. It's like openly admitting you associate "mental illness = evil/violence" when in fact mentally ill people are disproportionately victims of violence, not the perpetrators.