"The 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

This is correct. If they did manage to win those that helped them do so would be the first to be lined up against a wall because they would understand how they won. That understanding would be a threat to the regime and therefore need to be eliminated. But leftists cant think more than 1 move ahead because of their feels.