A society's mode of production (how goods are produced and who owns the means) forms the material base that shapes its legal, political, religious and cultural superstructure, not the reverse. Ideas and institutions are explained by the underlying economic relations rather than existing as independent forces.
The capitalist pays the worker only the cost of subsistence (the value needed to reproduce the worker's labor) while keeping the additional value the worker produces beyond that during the working day. This unpaid surplus value is the source of profit, so capitalist wealth rests on the systematic appropriation of labor the worker is not paid for.
Every society since antiquity has been divided into opposed classes (master and slave, lord and serf, bourgeois and proletarian), and the conflict between the owning class and the producing class is the engine that drives historical change. Under capitalism this reduces to two camps, bourgeoisie and proletariat, whose struggle points toward revolution.
Karl Marx, born 1818 in Trier, spent decades in exile and chronic poverty, living mainly in London and researching for years in the British Museum library while financially sustained by his collaborator Friedrich Engels. Several of his children died from want, yet he kept working on Das Kapital, whose later volumes Engels assembled and published after Marx's death in 1883.