All kidding aside... yes, they do believe that.
Liberals’ belief system centers around the assumption that people are basically good and respond to their environment with corresponding choices and behaviors. In other words, bad choices and evil behaviors are only a result of a bad environment, and if that environment were good, they’d act good. Of course, that good environment would have to be created, and of course, only those with this belief system would want to created such an environment. Those people would have to be given the power to do so.
The reason they are baffled, or in denial, when it is shown that this fails every time, is that it would require they give up on their assumption of human nature. And they don’t want to do that because that would logically and inevitably lead to “Mere Christianity” as the only solution to the human condition.
I have critized the “Noble Savage Myth” many times here, glad to see you know if it’s evil as well.