I, unfortunately, am not a person with a whole lot of intuitive insight into human nature. That said, I do always tend to mitigate the blame in cases by this by assuming that, since at this point he's not really arguing coherently, either he had a traumatic
personal experience that he's reacting to irrationally, or, literally, his mind is failing.
People should try to reason with him, though. And I mean persistently and challengingly. Unless and until it becomes clear that he's a victim of amyloid-beta plaque.