Karl Marx co-wrote the Communist Manifesto in 1848, and it should be understood from the perspective of rapidly-industrializing Europe. The lower classes in urban areas lived in extreme poverty. Everything is examined through the lens of class conflict. The bourgeoisie is said to exploit the proletariat, and this system will eventually crumble under a revolution of the proletariat.
From this perspective, clearly a) abuses dealt to the working class, c) poor conditions, d) employers taking advantage, and e) profit-sharing contributed to Marx's theories.