in late 1984 and early 1985, he was betrayed to the KGB. (CIA officials believe the informant was Edward Lee Howard, a one-time CIA trainee who had been fired after failing a series of polygraph tests. Howard, who defected to the Soviet Union after Tolkachev’s arrest, denied betraying the Soviet spy. Howard died in Moscow in 2002.)
Howard was hired by the CIA in 1980 and was later joined by his wife, Mary, where they were both trained in intelligence and counter-intelligence methods. Shortly after the end of their training and before going on their first assignment, a routine polygraph test indicated that he had lied about past drug use, and he was fired by the CIA in 1983 shortly before he was to report to the CIA’s station at the American embassy in Moscow.
It's hard to imagine how he'd get the right sort of info on such a critical spy.
If Legacy of Ashes, a history of the CIA, is any sort of indicator (and it is as a factual history of the CIA), the CIA has been (and probably still is) an absolutely inept institution with a tremendous amount of incompetance. I'm not quite this far but I'm getting there. I'm sure it will talk about this guy.
So the CIAs incompetence got him killed.