In chapter 17 of Wuthering Heights, six months after Catherine dies, Hindley dies as well. He locks himself in his room and drinks himself to death. There had been several trips to the lawyers for consultations regarding debts, of which Hindley had many.
I insisted on the funeral being respectable -- Mr Heathcliff said I might have my own way there too; only, he desired me to remember, that the money for the whole affair came out of his pocket.
So, how does Heathcliff get his money for not only the funeral but for Hindley's entire estate?
Heathcliff leaves Wuthering Heights in 1780 penniless, and he returns three years later as a man of great wealth. How did he get it?
Heathcliff says he was in the army. But, according to one source:
Between 1780 and 1783, Britain was involved in the American War of Independence which was drawing to its end while, on the Continent was the minor War of the Bavarian Succession. So there was no major war to benefit from, either as a Briton or a mercenary...
Alternative explanations for his remarkable rise are that he was involved in the slave trade (which did not end in Britain until 1807) and, alternately, just simple criminality. In my view, the slave trade would seem the best explanation. It would explain his reticence in revealing his past, his foreign pronunciation, and would also fit his character. (Heathcliff doesmention slavery in chapter 11 – "The tyrant grinds down his slaves and they don't turn against him; they crush those beneath them" – but this cannot be taken as proof of his past.)
So, Heathcliff becomes a mercenary slave master? Talk about subtext. This undeclared backstory changes our whole view of Heathcliff. It seems Nelly is quite in denial of Heathcliff's dark side. She is, no doubt, quite smitten with him.
So, Heathcliff pays for the funeral so that he can have Hindley's entire estate and become caretaker of Hareton, who should be a squire. Instead, he is but a common servant to his father's enemy. Why does Heathcliff do it? Could it be revenge? Heathcliff will raise Hareton the same way that Hindley treated him. So says Enotes:
Discussing Hindley’s affairs with his attorney, Nelly learns that Hareton is penniless. All of Hindley’s estate has been heavily mortgaged to Heathcliff. He grudgingly permits her to give Hindley a decent burial at his expense, but surprises her by refusing to relinquish Hareton to Edgar’s guardianship. Caressing the child roughly, he tells Nelly that raising a child might be amusing, and if Edgar insists on taking Hareton, Heathcliff will take Linton away from Isabella. Halted by this threat, Edgar does not press further. As a result, Hareton, who by birthright ought to be the neighborhood squire, is completely dependent on his father’s enemy—impoverished and friendless—and, most sadly, unaware of the injustice done him.
So, it seems that Heathcliff takes satisfaction not only in getting revenge against Edgar and Hindley but in taking a slave in Hareton. Not only does he pay for the funeral and estate with money made as a slave master, but he becomes a slave master of sorts within his own adopted family.