Charges of "class envy" have turned into a conservative weapon for keeping the jealous masses in their place. Conservative Christians deploy it in a similar effort to shut down debate: You have no right to ask how you have followed all the rules and still ended up in a deep economic hole; your job is to be grateful for what you have, and help the rich in hope that they might someday come into need of your services. If a Christian suggests there are other ways to look at the situation, like the justice of policies that have produced a system more and more rigged against people of modest means, the human nature card comes out: You just don't get it do you? We can't make this better. We have to accept the way people are.
I think it's actually conservatives who often fail to understand human nature. Their defenses of the free market tend to assume the fairness of the rules and that the "job creators" will play by the rules. They also assume the scientific order of markets. This is, it turns out, the kind of misplaced faith in human institutions (businesses, laws) and human actions (economic transactions, investments) that they often haughtily dismiss liberals for harboring. In reality, these are very shaky grounds on which to build a faith: capitalists will always choose the path of most profit regardless of the human cost; markets are, despite the oceans of free-market propaganda, wildly irrational and often grossly inefficient.
Most importantly, conservatives often twist human nature into more of an excuse for capitalism than it is. Humans are innately driven to satisfy their biological needs—violently if necessary—but they are not necessarily born with a relentless drive for commercial prosperity and extraneous wealth accumulation. People want and value what society tells them they should want and value. Some conservatives, it seems, are too myopic to see beyond the system they live under, where the relentless marketing of junk has created men and women with a thirst to acquire "stuff." The world is replete with people who are far less materialistic, acquisitive and driven by money than Americans. Someone who takes our country's grotesque distortion of human nature for reality is following Ayn Rand, not Jesus.