morality has never been easy to disentangle from history because everyone interfaces with material changes around them through a lens colored by superstructure. that means that in the application of marxism it’s always been impossible to keep them at arms length, and it’s counterproductive to try. e.g. here’s rosa

Thus, injustice by itself is certainly not an argument with which to overthrow reactionary institutions. If, however, there is a feeling of injustice in large segments of society – says Friedrich Engels, the co-founder of scientific socialism – it is always a sure sign that the economic bases of the society have shifted considerably, that the present conditions contradict the march of development. The present forceful movement of millions of proletarian women who consider their lack of political rights a crying wrong is such an infallible sign, a sign that the social bases of the reigning system are rotten and that its days are numbered.

or lenin

You have to build up a communist society. In many respects half of the work has been done. The old order has been destroyed, just as it deserved, it has been turned into a heap of ruins, just as it deserved. The ground has been cleared, and on this ground the younger communist generation must build a communist society. …

The entire purpose of training, educating and teaching the youth of today should be to imbue them with communist ethics.

But is there such a thing as communist ethics? Is there such a thing as communist morality? Of course, there is.

or marx himself

Shame is a kind of anger turned in on itself. And if a whole nation were to feel ashamed it would be like a lion recoiling in order to spring.

later marx would be alarmed by this subjectivity and try to set at least his historical method on transhistorical footing, but marxism is more than philosophers interpreting the world; in fact as praxis it aims to repair “the complete rift between books and practical life”

so I see at least a couple of pieces which frustrate attempts to put marxism into a little economic box:

• nobody is really a material interest maximizing robot, making moral sensibilities a key aspect of class struggle
• marxism acknowledges and celebrates the human drive for progressive change and expects the proletariat to finally seize “real possibilities of human freedom and happiness” (which necessarily includes building a communist culture that encompasses all spheres of social life)

lastly just to touch on your stalin quote, check out mao’s review of that book, he calls stalin’s blindness on this issue “almost altogether wrong”