I think he held delusional beliefs, so I don't think insane is an unfair word, even if it's not thought of as a nice word.
Being paranoid and anti-social would be enough.
Although I suppose in society there are acceptable and unacceptable delusions.
Solskytz, as a Jew and a proud American, Fischer's antisemitism and anti-American sentiments, all of which augmented greatly during his post-WC isolation, sadden me, but there are bigots and haters around every corner. He had challenges in his life that affected him in many ways and this was apparently one of the results. I've never considered him insane or mad or even batty, if I want to get personal, but definitely paranoid and anti-social. He never killed anyone or even hurt anyone that I ever heard of, so he's miles ahead of the people I do consider crazy.