In chapter 9 of Wuthering Heights, Catherine reveals that Edgar Linton asked her to marry him. She tells Nelly that she accepted the proposal. The woman also expresses her love for Heathcliff. However, she also mentions that “It would degrade me to marry Heathcliff.” These are the exact words Heathcliff overhears. After that, he decides to leave Wuthering Heights for three years. During this time, Catherine marries Edgar.
Detailed answer:
At one point in the book, Catherine stays with the Lintons in Thrushcross Grange for five weeks. During the time spent with the family, Catherine changes. Influenced by Mrs. Linton, she turns into a noble lady with good manners. Catherine returns to Wuthering Heights, barely recognizable. Hindley and the rest of the residents are pleasantly surprised by the positive changes. Heathcliff is the only one to whom the changes seem unpleasant. He realizes that Cathy is no longer the wild girl he knew. She is now very different from his childhood friend. Different from the girl he roamed the heather fields with before.
In contrast to Heathcliff, Edgar is a nobleman. He is gentle and kind. Influenced by Edgar and his family, Catherine changes as well. She starts openly criticize Heathcliff for his ignorance. Despite her love for Heathcliff, the woman decides to marry Linton. Deep down, she has dreams of using Edgar’s money to rescue Heathcliff from the oppression of Hindley. Heathcliff considers her desire only a betrayal. When he overhears Nelly talking about it, he and immediately leaves Wuthering Heights. At first, Catherine is devastated. Nevertheless, she still marries Edgar and moves to his house at Thrushcross Grange.
The misunderstanding leads to Cathy’s illness, madness, and eventual death. It also causes Heathcliff’s demonic transformation. He vows to avenge everyone who was willingly or unwillingly involved in his tragedy.