If there is such as thing as human essence and we can discover what it is then that will go a long way towards developing a moral theory.
Human nature is part biological, part social and not religious. Religion is something to be explained rather than believed. This includes modern religions such as Nature worship (currently popular) and Marx worship (currently marginalised).
Humans have both needs and powers. Obviously, it follows that we are both needy and powerful and both of these aspects of being human need to be explored further.
The biological and social parts are connected or interact dialectically. It would be an error to see them in isolation from each other.
Fundamental biological needs include eating, drinking, habitation, clothing, sexuality ...
Biological and Social. Humans produce their own existence / material life through social labour. Our biology allows this, eg. Opposable thumb, upright posture frees the hand, large brain. This separates us from other animals. Compared to other animals we are self conscious and wilful to a qualitatively different degree. Although we originate as part of nature, with our social labour we oppose nature. Our productivity is also imaginative. We imaginatively and self consciously transform nature and in that process also transform ourselves. This is a teleological process. Humans imagine new forms of the material and self and then through social labour bring that imagination into reality. This is human essence.
THE GUIDING MORAL PRINCIPLE
The guiding moral principle is to do whatever is required for the human flourishing of rich individuals, to dynamically expand human powers for all humans. Human flourishing is not original to Marx but Marx built on the best available ideas that came before him, those of Aristotle.
Marx and Engels were more aware than Aristotle about the role of social labour in this enrichment process. After all, Aristotle lived in a slave society. Refer Engels, The Part Played by Labour in the Transition from Ape to Man. In communist society there would not be a division of labour based around the supply and demand of the labour market. In a world where production for the needs of all is established then each individual would be free to pursue their own perceived interests.
The philosophical stance here is to investigate what is distinctive about humans (biologically and socially) and from that basis to articulate what a good or rich life is.
Be clear about where our moral principles come from. Being determines consciousness. Matter is philosophically prior to ideas.
The theory is philosophically materialist. It starts from real people and real conditions. It ascends from earth to heaven, not descends from heaven to earth.
But, once we are in heaven how do we get back down to earth again? The only way is to make a detailed study of society in all its aspects. Mode of production, division of labour, social classes, Is there a surplus and who controls it?, the history of knowledge, current issues, individual self knowledge. There is a lot to know! The desirable actions that promote the best human nature at any point in history depends on the depth and perspicacity of such an analysis.
CAPITALISM and ALIENATION
Capitalist limitations. For the capitalist, because they own the means of production, the workers life activity becomes a mere use value. In general, workers have no direct stake in the products they produce. Temporary niche solutions may be possible for individual workers but overall work loses it human character. In class society, the economy operates as a thing more or less outside of human control. If there is no profit to be made then production grinds to a halt. There maybe poorly understood economic laws. But the best that could be said of capitalism is that it is a highly unstable system in which the future well being of the workers who make it is uncertain and problematic.
Capitalism gives labour a bad feel (alienation) and production a bad name. Under capitalism humans are alienated from their essence, their living social labour, since the capitalists own the means of production and determines which products are made and who owns those products.
The capitalist economy is an unstable monster, poorly understood, difficult to manage and continually spinning out of control. Workers are alienated from the products they produce. Creative people who produce things of beauty (some artists, some writers, some teachers etc) are often not seen or appreciated as typical workers, rather they are marginalised workers looking for a niche to survive in a system that systematically undermines them. Or a handful may become megastar celebrities who play a significant role in entertaining the masses. Moreover, many believe today that capitalist production is despoiling the environment at an alarming rate. I think there is some truth to this latter charge, although I also see talk of environmental Armageddon as exaggerated and a distraction from the main wrongs of our society.
These issues in combination (production for profit not human need) give production itself a bad name. Human essence, social labour, life's prime want, is reduced to being a wage plug, without a real say in the overall progression of society.
Rather than saving the planet (the current “left” mainstream zeitgeist) we need to focus more on how to liberate the social productive forces, human essence, in all their real power and beauty. A power and beauty which is obscured by the ugliness of capitalism.
The natural world is the world created by humans, who are part of nature, as well as the world that existed before humans. The natural world is not “green” insofar as that suggests a world not touched by humans. Such a world no longer really exists on Earth. In a post natural world (aka the anthropocene) our needs will be created more by what we make than the natural world that exists independently of what we make.
As society evolves our tastes, including our basic biological tastes, become more sophisticated: “the forming of the 5 senses is a labour of the entire history of the world” (source)
THE SCOPE OF MORAL THEORY
A moral theory has to somehow account for all human moral thinking, good and bad, angelic and evil, noble and perverse, optimistic and pessimistic. But Marx's moral theory is (intentionally?) thin. It does not claim or suggest that humans are any of essentially selfish, altruistic, competitive, fallen, vicious etc. Is this a feature or a bug? In my view Marx is right about the essentials but there is a lot of stuff that is not covered. Marx analyses the deep structure of capitalist society but there are important issues that lie more on the surface (eg. the dark and deep emotions such as love, grief, anger) that strongly motivate individual actions but are left hanging. Hence, many people find that other moral philosophers and novelists address their needs more directly.
UTOPIANS and OTHER ALTERNATIVES
Utopians make the error of promoting general moral principles in the abstract, without regard to the current real state of society, without assessing the social forces at play. They are not realistic. Mere moral persuasion in favour of a better society is inadequate / doesn't work.
There are many alternative moral theories. For example Plato (Iris Murdoch provides a modern interpretation), Stoic, Christian (various branches), Kantian, Utilitarian (Bentham and JS Mill provide different interpretations), feminism / women's liberation, Buddhism (meditation and mindfulness are currently popular), existentialism, libertarianism, animal liberation, Sufism (adopted by Doris Lessing after her disillusion with communism), pragmatism (Dewey, Putnam), the liberal Capabilities approach of Amartya Sen and Martha Nussbaum.
All of these need to be critically examined since what is correct only emerges clearly from a critique of such alternatives. At this stage I would say that none of these alternatives share with Marx the view that human essence is the conscious production of our existence / material life through social labour. Moreover, they tend to be indifferent to the analysis that the main current problems are generated by capitalism.
INDIVIDUALS and SOCIAL CLASS
Humans are self conscious, intelligent, purposive, active, self directed. But this doesn't mean we can negate the so called "external world" (only external to humans, who are a part of nature, so not really external to nature in that broader sense of the word) or history.
Human individuality (as distinct from herd or tribal mentality) emerges historically from the bourgeois revolution against feudal relations (when it was “natural” to obey a preordained superior such as a lord or king). Herds are not good at shopping, whereas individuals are. But just as individuality emerges strongly in the capitalist era, you would expect it to also change dramatically in a post capitalist society.
In class society, social class is a more important determiner of who we are than individuality as such. Individuals pick their personalities, interests, work etc. from what is available socially (including the cutting edge, futuristic and off beat, quirky trends) at the time. The idea that we are free, autonomous individuals is more part of capitalist mythology or ideology than reality.
Morality is historically contingent. What is moral in one historical period becomes immoral in another. The central issue is doing whatever is required to maximise the human flourishing of rich individuals in the given time and place.
For example, in the French revolution the rising bourgeois class overthrew feudal relations, got rid of divine rule by the King etc. In that historical period bourgeois right coincided with the needs of the proletariat as well. But at a later date the bourgeois class held things back, became reactionary, used social labour for their own ends, promoted an economic system which went through periodic crises and still does. At that point the revolution to continue human liberation and the liberation of the productive forces must be picked up by the proletariat, sooner or later.
Given the views expressed here about ontology (materialists need to deeply investigate reality) and history (morality is historically contingent) it follows that to work out the best moral - political actions requires some hard work. No one said it would be easy.
TRANSITION FROM CAPITALISM TO COMMUNISM (SOCIALISM)
The productive forces developing within bourgeois society create the material conditions (preconditions?) for the solution to the problem of the antagonism of the individuals social conditions of existence. Big is beautiful, not small is beautiful (the latter from EF Schumacher). Not because capitalism is beautiful but because big, centralised production prepares the way for socialism.
Marx is grounded, not utopian. In The German Ideology, Marx rejects the idea of communism as "an ideal to which reality will have to adjust itself", rather he sees it as "the real movement which abolishes the present state of things”
This is pretty much the opposite of what most people today believe about communism, that it is idealistic and unrealistic.
From a moral perspective the aim is to bring together social being (human existence as it is) with social essence (human existence as it ought to be). As the contradiction between the individual and the social diminishes then the need for morality to maintain social cohesion would also diminish. All the conditions for rich individuality would be met by society. Eventually, morality might disappear altogether. If everyone's needs were being met through the basic social structure then wouldn't concepts such as selfishness or altruism lose their meaning?
SOME THINGS MISSING FROM THIS ACCOUNT OF MARX'S MORAL THEORY
There are many important issues missing from both the theory and practice of Marxism in this account. I have a preliminary list but will leave that to another time. No doubt if you have read this far you are both interested in this topic and will have your own unanswered questions. This will require far more discussion.
I have done a lot of reading on this topic but won't attempt a detailed bibliography at this stage. But I will mention one reference which to me is a stand out, a PhD thesis by Vanessa Wills titled Marx and Morality (pdf 269pp) who has read and understood all of Marx IMHO.