He's not a lone nutter. He believes the KGB/Soviets were behind the assassination. Or a group of them; the so-called "inner" KGB. That is to say, he's a conspiracist.
If you're referring to me, you've misread me.
My believing that a probable KGB mole by the name of Bruce Leonard Solie (look him up) in the CIA's mole-hunting Office of Security sent (or duped his confidant, protégé, and mole-hunting subordinate, James Angleton, into sending) Oswald to Moscow in 1959 as an ostensible "dangle" in a planned-to-fail hunt for "Popov's U-2 Mole" (Solie) in the wrong part of the CIA -- which mole hunt lasted nine years, tore the CIA apart and drove Angleton nuts -- doesn't necessarily mean that I believe the KGB was behind the JFKA (although there are some tantalizing clues that it was).
The "Inner KGB" that you alluded to (Department D of the First Chief Directorate -- foreign intelligence -- today's SVR) was created in 1959 when the Kremlin realized that the USSR and the Warsaw Pact couldn't defeat the U.S. and NATO militarily and decided to get us to get us to tear ourselves apart, instead (Can you say Donald J. Trump?). General Gribanov of the more secretive Second Chief Directorate (domestic intelligence and overall counterintelligence -- today's FSB), not to be outdone, set up his own Sun Tzu-based Department 14 in the SCD, and, as soon as CIA's spy Oleg Penkovsky had been "trapped like a bear in its den" in such a way as to not reveal who in U.S. or British intelligence had betrayed him within two weeks of his April 1961 recruitment, sent Aleksei Kulak (J. Edgar Hoover's shielded-from-CIA FEDORA) and Dmitry Polyakov (TOPHAT) to the FBI's NYC field office to "volunteer" to spy for it at the U.N.
Six months after KGB Major Anatoliy Golitsyn's 12/15/61 defection to the U.S., Gribanov sent false-defector-in-place Yuri Nosenko to the CIA in Geneva to discredit what Golitsyn was telling Angleton about penetrations of U.S. Intelligence and the intelligence services of our NATO allies, which intel, unfortunately, Angleton was naively sharing with the aforementioned Bruce Leonard Solie (do remember to look him up) just as he'd shared intel with his earlier father figure, Kim Philby.