Every human being alive would benefit from therapy. But that in and of itself is not a diagnostic criterion nor does it mean someone has a mental illness.
Furthermore, the existence of a mental illness in an individual who commits a violent act does not by any means dictate that their mental illness itself can be attributed as the cause.
I think you and others are getting hung up on the fact that this individual was not acting rationally and clearly had unhealthy (likely some delusional) beliefs. But that is due to an indoctrination into that conspiratorial mindset with expectations of persecution and violence and feeling justified in 'defending' themselves.
You can argue that this man clearly had dysfunctional thinking and I would agree with you. But this dysfunctional thinking is caused by misinformation (Fox, OAN, Newsmax) and echo-chamers on social media that rile these individuals and put them in a perpetual state of feeling like their ideals, culture, and safety are being threatened.
The misinformation in media, lies of corrupt politicians, and toxic delusional echo-chamers are the causes directly responsible for the culture we live in right now, not mental illness. And that is what it has become; a culture of extremism. Not mental illness.
This is similar to how people try to blame mass shooters on the big bad "mental illness" when there are so many actually relevant factors in play, including the media's role itself in inspiring others thru eternalizing (and thin their minds enshrining) the mass murderers with their news coverage, plastering their names and pictures everywhere.
You can acknowledge someone is mentally unwell, but that does not mean that they acted the way they did due to mental illness. One problem I believe perpetuates the myth that mentally ill individuals are violent is the comorbidity of mental illness and substance use. Unlike mental illness, substance use is a predictor of violent/aggressive behavior... Substance use also exacerbates symptoms of mental illness. So when someone sees a homeless person who's obviously mentally ill acting aggressively, they often think it's the individual's mental illness causing the aberrant behavior. But in reality, the substance use is actually what is responsible for the violent behavior as well as the exacerbated symptoms of the individual's mental illness.
