We have been critical of marxism’s lack of an ethics, but, as with all criticisms of Marx, there is a literature denouncing this. But I think such hyper-subtle defenses tend to confuse the issue and make marxists simply persist in their confusions. This is absolutely typical of marxism: every criticism has been defended with sophistical bastions of jargon, and this never convinces outsiders and yet confuses ad infinitum the insiders and true believers.
Before being too critical it is important to ask if religionists, e.g. readers of the Old Testament, have an ethics? Sorry, but the Old Testament is almost an insult to the modern intelligence. However the great saga of the Bible had an immense influence inducing a social/moral transformation for all the peoples of the Roman Empire.
I think we should say marxism has no ethics in the sense of comrade critics like the Kantian socialists. Their views in turn may need critique (and one of its main adherents Bernstein ended up in a supposed betrayal to the right, but I don’t think this was due to Kantian ethics).
The Kantian socialists focused on the Kantian theme of the ‘republic of ends’, a brilliant strategy in the Kantian canon.
Basically we need a human definition that includes a creature with a will and a free agency and a core ‘module’ of reason that stands in some relation to the deduction of moral results, I won’t say categorical imperative. Now this Kantian stance may itself be a problematical, but it at least makes the principle that human psychology is not to be subjugated to physical causality thereby turning man into a pseudo-moral robot. Even it Kant’s reasoning is flawed it shows the way to an in principle ethics.