Marx wrote the Manifesto for the Communist League as a propagandizing pamphlet. It's not even close to a treatise on political economy, which he will write later after the failed revolutions of 1848, and therefore not the best text to read to understand Marxist thought. He was operating out of London with Engels just before the events of 1848 broke out. By this time in London and other major cities in Western Europe, the aristocracy of the feudal mode of production had been waning, converted into or replaced by business owners known as the bourgeoisie. This class, as owners of the means of production, required a class of workers that have only their labor to sell in exchange for wages. The two classes Marx and Engels (among many many others) were interested in were the bourgeoisie and the working class, not the aristocracy. If the working class is the negation of the bourgeoisie, then socialism is the negation of that negation; it's the destruction of both classes, like matter and anti-matter colliding. The end of feudalism, like the end of capitalism, are historical events. So the working class and the bourgeoisie are situated in a particular time and place. The working class, then, is not the subject of all economic class struggle. In feudalism, for example, the main struggle is between the aristocracy and the peasant classes.