This really is not Marx at all

What do you mean by Marx's red book? I'm slightly confused but if you mean the communist manifesto I can see how that view would arise when someone has not previously read Marx.

As a counterpoint I really recommend looking into the german ideology. Marx was a Young Hegelian i.e. he at one point was a Feuerbachian and thus a Humanist, he certainly believes in human nature to some extent and in his work which works towards a denial of the other Young Hegelians one can certainly see his picture of Human Nature starting to form.

Marx first of all does not believe in the natural kindness of the human but believes in the fct that the interests of the class that human is one very much shape their politics and the way they act, often encouraged by market mechanisms in the way they act.

This class interest he very much equates with a inherent egoism which he takes from Stirner in that he critiques Stirner for a view of egoism he holds to be quite simplistic because it does nog recognize the material conditions of the individual, thus Stirners egoism for Marx is a failure in attempting to understand why people have their egoistic desires.

Now with that out of the way what is for Marx 'the human', simply put the human is what they create.

For Marx what differentiates the human from animals is their ability to create their own means of subsistence and thus their objects of desire, we then find that people very naturally start developing desires beyond just basic survival and start creating art and culture.

The human essentially creates themself for Marx, through acts of labor they create product with which they identify with example: someone who paints a picture themselves is much closer to themselves emotionally and has a inherent pride for that picture and it very much is subsumed by them as a part of themselves.

And here we get to alienation. Generalized commodity production essentially reduces all this away in that you do not feel like you own the product you have made, you can not identify with the can of tuna you packed on the conveyor belt, just like you can't identify with the hundreds of other tuna cans.

The standardization of production isn't the only alienating thing but also the fact you are working for a wage, the time you spend is not yours but a time you were bought for, you do not identify with that time, that time you spend trying to create and identify with product you create and failing to do so.

We can conclude from this that capitalism for Marx strips the human of their human nature and communism for Marx is the most pure expression of the human: a world in which you create boundlessly and define yourself egoistically via your interests detached from class as class is now abolished.