According to Robert Service
[Friedrich Engels] was one of those coupon-snipping rentiers that he and Marx subjected to withering contempt in their pamphlets.
Their outlook, being rooted in socio-economic determinism and philosophical amoralism, inured them to pangs of conscience about the creators of their comfort: the hundreds of Mancunian workers toiling for miserable wages in Engels’s factory that made its patented “Diamond Thread”. Marxism’s co-founders believed that the greater good of humanity was served while they had optimal leisure to elaborate their “correct” theory of communism.
According to The Economist
Marx had started work on “Das Kapital”, but there was a problem. He had by then acquired an aristocratic German wife, a clutch of small children and aspirations for a comfortable bourgeois lifestyle, but no means of support.
Engels (whose name resembles the word for “angel” in German) offered an astoundingly big-hearted solution: he would go back to Manchester to resume life in the detested family cotton business and provide Marx with the money he needed to write his world-changing treatise. For the next 20 years Engels worked grumpily away, handing over half his generous income to an ever more demanding Marx.