"[T]he useful idiots, the leftists who are idealistically believing in the beauty of the Soviet socialist or Communist or whatever system, when they get disillusioned, they become the worst enemies. That’s why my KGB instructors specifically made the point: never bother with leftists. Forget about these political prostitutes. Aim higher. [...] They serve a purpose only at the stage of destabilization of a nation. For example, your leftists in the United States: all these professors and all these beautiful civil rights defenders. They are instrumental in the process of the subversion only to destabilize a nation. When their job is completed, they are not needed any more. They know too much. Some of them, when they get disillusioned, when they see that Marxist-Leninists come to power—obviously they get offended—they think that they will come to power. That will never happen, of course. They will be lined up against the wall and shot."
— Yuri Bezmenov
Here is an introductory paragraph about Bezmenov from this website:
“Yuri Bezmenov has been dead for decades, but his words have renewed attention. The former Soviet journalist and KGB operative defected to Canada in 1970 and adopted the name Tomas Schuman. Although he died in 1993, his writings, lectures and interviews have found new life in the internet era. They even formed the trailer for the Call of Duty Black Ops Cold War video game. There’s a good reason for his renewed fame. Bezmenov speaks authoritatively about the horrors and methods of a totalitarian Marxist state.”