If it doesn't excuse him, then there's no need to bring it up.
But in general, people have this habit of using one-off videos of racists' outbursts to try and diagnose them as having something medically or physiologically wrong with them when A.) Actual psychological and medical professionals don't do this, nor do they instruct laymen to do so, B.) It plays into wider narratives that racism isn't a big deal and that black people are overreacting, because racism can always be explained away by some extenuating cause, C.) It centers sympathy on the racist because suddenly they're a mental health case or a drug addict, D.) It stigmatizes those with actual mental health issues and struggles with drug abuse by conflating them with bigots, and E.) Other forms of bigotry do not get this treatment. No one looks at an incel or a transphobe or an anti-Semite and wonders what their drug history or if they're bipolar. They just call them bigots. Racists, specifically anti-black racists, deserve the same lack of the benefit of the doubt.
So when you do this, when you idly speculate about a racist's health history as a reason for a racist ourburst, you are being harmful, and no it doesn't matter that you said "it doesn't excuse him," no more than it matters when someone prefaces a statement with "I'm not racist, but...."
Just fucking stop doing it.
if he was making the same rants about gays, jews, trans or women there'd be the exact same speculation that he is a mentalist, because he's acting like a mentalist.
I know Nepenthe knows literally fuck all about anywhere in the world, but that dude was well outside the societal norms of people on the underground and squarely in the realm of mentalism, of which there is no shortage of nutters in london.
Hence everyone else on the tube treating like him a mentalist. Because thats what he was coming across as
