Guilty by Reason of Insanity
Bobby Fisher did not appear to be psychotic. He didn't, apparently, hear voices or see visions. Nonetheless, his self-destructive pattern of behavior was clearly driven by paranoia combined with an apparent inability to judge or appreciate other people's points of view, and a fierce sense of entitlement. Somewhat similar peculiar behavior seems to have been found in a few crazy geniuses - Kurt Goedel, Grigori Perelman, and Alexander Groethendieck, for example.
My guess is that the underlying condition is something like Asperger's syndrome, but most with Asperger's don't appear to suffer the same kind of personal disintegration.