It means drinking the poison of silent consent to malfeasance, incompetence, and cover-up. We are reminded of CIA's attempt to change the psychological profiles for the classes of 1979 (our class) and 1982, from "go along little gray man" to "innovative self-starters." Half of each class quit within five years--they would not drink the Kool-Aid. The other half generally lasted another four to five years, they had a higher tolerance for poison.
Now let's dissect that for a moment. What it really means is that the "good part" of CIA, the personnel system seeking the best and brightest, had the right idea. What it also means is that the "bad part" of CIA, the entrenched ego-maniacs at the top of the Directorates of Operations and Intelligence (cowboys in one, weasels in the other), could not tolerate innovative self-starters, and drove them out of the system--in the case of the strongest internal critics, using "Fitness for Duty Physicals" to attempt to intimidate them with the threat of being declared insane or unbalanced and thus unfit for any federal employment.
When an entire management chain has "drunk the Kool-Aid" and is prepared to kill anyone that is not willing to go along, it is time for wholesale change.