I'd say there's two different Leftist groups pushing a cognitive dissonance. I'll call them the Early Leftist and the Late Leftist. Liberalism pre-supposes that people are innately and naturally good, and that civilization corrupts people as a sacrifice of living together. The Early Leftist accepts this premise, and as such, decides society must be built in such a way to stop the corruption of civilization. They create policies, institutions, structures, and social pressures to try and stop the benighted people of the world from falling into the corruption of civilization and evolving into a higher plane of being, like the Leftist. However, over time, every one of these policies fail because several Liberal assumptions are false. The Leftist finally comes to the conclusion that: clearly humanity isn't innately good, and they do not fit into the Leftist ideological utopia. And instead of asking if they misunderstood something, they declare that humanity itself is broken and irredeemable, which justifies unlimited force in order to cultivate the New Socialist Man. Men are not born good... right now. So, a new man must be created in order for the society to work. Afterall, if civilization can corrupt men, then surely men can be corrupted into doing the right thing. It is at that point where the Leftists acquiesce to just killing all the benighted people, and using unlimited force to groom the new man into existence.

So, the Early Leftist believes that people are genuinely born good. But the Late Leftist learned that people are not genuinely born good, and are prepared to force them to become good, as long as "good" means "meeting the requirements of a Leftist ideology".