Daniel Little writes about Marx’s concept of capitalist injustice with characteristic seriousness. However, a lecture of John Rawls is not going to be a very good guide. (Nor, for that matter, is Brad DeLong’s commentary.)
The debate about whether Marx developed a scientific or moral critique of capitalism goes back a long way, and is not a very fruitful way of posing the question. Every theory of political economy—or, as with Marx, critique of political economy—has both scientific and moral (or, if you prefer, Utopian) dimensions. Thus, for example, neoclassical economics is based on a view of the world in terms of supply and demand (and, behind them, given preferences, technology, and resource endowments), which affects and is affected by certain notions of justice (such as voluntary individual contracts) and Utopian views (such as the efficient allocation of scarce resources). Other economic theories are based on analogous (but quite different) notions of justice and imagined utopias.
The same is true of every social formation: each has its own standards of justice (actually, both hegemonic and nonhegemonic standards of justice). Thus, for example, according to the dominant ideology of capitalism, slavery is unjust (because it is based on human chattel) and capitalism is just (because it is based on free individual decisionmaking, including decisions over one’s body).
What Marx did is develop a critique—of bourgeois thought and capitalist society—starting with bourgeois notions of justice. Thus, for example, in developing his labor theory of value, Marx started with the idea of the exchange of equivalents (a bourgeois notion of fairness) and then “discovered” surplus-value (a social theft of surplus labor from the direct producers), which violated bourgeois norms. That discovery was, in turn, predicated on (as it informed by) the idea of non-exploitation (which, as Marx explained in the “Critique of the Gotha Program,” does NOT mean the direct producers get all the labor they perform).
There are two important points here: First, Marx’s critique of political economy does have moral and Utopian dimensions. Second, the moral critique of capitalism (as social theft) does not, in itself, give rise to a communist morality (since it is a very different kind of society, in which exploitation is eliminated). The key, at least for me, is Marx’s insistence (as in his 1843 letter to Ruge) on the
ruthless criticism of all that exists, ruthless both in the sense of not being afraid of the results it arrives at and in the sense of being just as little afraid of conflict with the powers that be.