He accuses her of treating him infernally and vows revenge, hinting he'll exploit an intimate secret.

“And would Mr. Linton approve of his sister marrying that man?” I inquired.

“Mr. Linton should approve,” returned my lady, decisively.

“He might spare himself the trouble,” said Heathcliff: “I could do as well without his approbation. And as to you, Catherine, I have a mind to speak a few words now, while we are at it. I want you to be aware that I know you have treated me infernally—infernally! Do you hear? And if you flatter yourself that I don’t perceive it, you are a fool; and if you think I can be consoled by sweet words, you are an idiot: and if you fancy I’ll suffer unrevenged, I’ll convince you of the contrary, in a very little while! Meantime, thank you for telling me your sister-in-law’s secret: I swear I’ll make the most of it. And stand you aside!”