Is human nature a problem for Marxists?
So I am very broadly speaking familiar with the idea that before the scientific phase of Marx's work he was more humanist, argued that human nature was being corrupted by capitalism. We are alienated from our species being, in so far as we are alienated from the fruits of our labour and other labourers.
I'm used to libertarian socialists saying that human nature is extremely malleable: that, given a social revolution, men and women will have a different, better, nature.
But how do we get from "this" - no-one gives a fuck, communism trough the barrel of a gun, if at all - to "that" - the vast majority of humanity, the working class, acting in its universal interests to create a better world for every proletarian?
Is this about economics, vanguards, or just some theoretical trajectory? How do we get to solidarity etc., given that we are almost entirely atomised by the economy and society?
I'm especially interested in answers which do not draw from the mass murder and failure of e.g. russian revolution (stalinism etc.) as a good, inspiring, turn of events, but seek revolution via the broad support and struggle/action of billions.
I do not consider stalinists communists. I consider them wannabe cops.