I was taught that factory workers in the north had it worse than slaves

In Marx's "Theories of Surplus Value" which he never published while he was alive, but was instead compiled from his notes by Kautsky, and then later Riazanov, he called out 1700s reactionary anti-capitalists like Linguet who made these kinds of arguments.

Linguet however is not a socialist. His polemics against the bourgeois-liberal ideals of the Enlighteners, his contemporaries, against the dominion of the bourgeoisie that was then beginning, are given—half-seriously, half-ironically—a reactionary appearance. He defends [...] slavery against wage-labour.

(Linguet was guillotined by the Jacobins lol)