Unfortunately, Heathcliff has overheard the conversation. He did not listen anymore after he heard Catherine remark that she wouldn't want to marry Heathcliff. He was not able to finish Catherine's speech talking about her love for him. I think that crucial moment was the turning point of Heathcliff's revenge.
It came to a point that Heathcliff ran away from Wuthering Heights. He was absent for three years. He went back to Wuthering Heights a changed man. His countenance was as dark as ever but he learned how to carry himself as a civil person. He has accumulated a mysterious sum of money which the author did not elaborate on anymore.
In that part of the novel, we come across clues of Heathcliff's plan of owning Wuthering Heights then we see that he too wants to own Thrushcross Grange. He becomes a scheming person using every resources he can to fulfill his goal.
We can see Heathcliff as cold-blooded. He went back to Wuthering Heights where Hindley remains a drunkard because of the loss of his wife. Hareton, Hindley's only son became Heathcliff's way of revenge to Hindley. He spoiled the little boy and raised him with no values and manners. He was not educated. Hindley was no longer financially capable to support the household and he depends on Heathcliff who had money. That is the ironic twist of things for Hindley.
Catherine was still torn between the two men that loved her deeply. One was the perfect husband for her, the other his lifetime passionate lover. I guess it must have driven her mad. Edgar being very devoted to her, allowed her freedom to correspond with Heathcliff in a friendly basis. I think he's also afraid of the possible outcome if he deliberately separate Catherine from Heathcliff. He resents the idea, for sure.
Being the one in charge in Wuthering Heights made him like another master of the house. He's assured of owning it because Hindley being inclined to gambling and alcohol was full of debt. He was concerned to the well being of his only son, Hareton. He had no choice; Wuthering Heights was as good as Heathcliff's.
With Wuthering Heights assuredly on his hands, Heathcliff plans in securing his ownership over Thrushcross Grange. His obsessive mind has put his mind into the fulfillment of owning the houses of those whom he hated. To get Thrushcross Grange, he found it prudent to be a part of the Linton family.
It was convenient for him that Isabella has developed an attraction to him. Catherine was against this. She tells to Isabella not to be tricked into Heathcliff's trap. However, Isabella is quite young and quite stubborn that she insists that Heathcliff was truly in love with her. She said that Catherine was just jealous, and partly, I think that it is true that Catherine is not really doing this just for Isabella's sake but also for hers. Catherine managed to tease Isabella by telling Heathcliff that she was in love with him in her presence; she was held by Catherine so she couldn't run away.
After she scratched Catherine's hand and walked away, Heathcliff expressed his interest to marry Isabella for her money and when she's his wife, he plans on beating her for having a mawkish and waxen face.
In chapter 11, we experience a very important scene where Heathcliff is seen by Nelly kissing Isabella outside Thrushcross Grange. He is scolded by Catherine when he came in. Heathcliff replied that he had the right to do whatever he wanted because she was already married; and that she can torture him in any way she liked but she should let him also do so in his the same style. We can see that there is an implication that Catherine is jealous of the affair. Edgar comes in the picture but was instantly humiliated by both Heathcliff and Catherine. He was simply ignored by Heathcliff. In this scene, we see how cowardly Edgar Linton is. Though Heathcliff and Catherine may behave and dress as refined and civilized as him, they were still wild-mannered and violent. Edgar was not accustomed to violence, he prefers a civilized and genteel was in handling difficult matters. He hits Heathcliff in the back and runs away to get reinforcements.
This event distressed Catherine so much that she decides to break her own heart to break the hearts of those who loved her. We can see here that she is also scheming and shrewd. She is willing to sacrifice her mental and physical well-being for the sake of her objectives thus she faked her health to be failing.
Long story short, Isabella being so infatuated eventually eloped with Heathcliff and became mistress of Wuthering Heights. Almost instantly after the wedding, we learn that Isabella realized of how grievous her mistake was in marrying Heathcliff. She was disowned by his brother who apparently did not enjoy the fact that his sister married his enemy.
