" The central argument of Capital is that the motivating force of capitalism is in the exploitation of labour, whose unpaid work is the ultimate source of surplus value and profit. "
Old soviet joke:
Under capitalism, man exploits man. Under socialism, it's the other way around!
Myself, I always exploited my employers by extracting money from them.
Marx's labour theory of value is only a theory of where profits come from and is largely rejected. Return on captial investment is not restricted to factory owners. All workers get a share of the surplus value and have the option to increase that by investing.