Forget about capitalism for a second. Did Marx think slavery was a just system? Feudalism? I'm pretty sure he thought they were unjust.
But, suddenly, here comes capitalism, and now it's all different? Marx speaks of workers being embezzled, profit as theft and like tribute exacted by overlords, etc., in extremely unflattering terms. Just remuneration/distributive justice underpinned his proposals for remedying capitalism.
-B.
Michael Perelman wrote:
"Marx was trying to downplay the reformers who based their program on morality. [...] In short, he was denying justice as a basis for political organizing, calling for what he considered to be a scientific basis.