Although Isabella casts Heathcliff as a romantic, Byronic hero whose hard exterior masks a tender heart that loves her, Heathcliff marries her to get revenge on the Lintons. He especially wants revenge on Edgar Linton for despising him and for marrying Catherine. He does not consider him a worthy husband for her and resents deeply his attempts to keep Catherine apart from him. He also despises Isabella as a weakling.
Isabella discovers that Heathcliff is not what she thought almost as soon as they elope. As she writes to Ellen:
The second question I have great interest in; it is this—Is Mr. Heathcliff a man? If so, is he mad? And if not, is he a devil? I sha’n’t tell my reasons for making this inquiry; but I beseech you to explain, if you can, what I have married . . .
By time she writes, Isabella has realized that Heathcliff hates her and will continue to treat her cruelly. She writes:
I do hate him—I am wretched—I have been a fool!
When Ellen goes to visit them at Wuthering Heights, Heathcliff talks to her about his marriage in front of Isabella, explaining what happened and his true feelings:
"She abandoned them under a delusion," he answered; "picturing in me a hero of romance, and expecting unlimited indulgences from my chivalrous devotion. I can hardly regard her in the light of a rational creature, so obstinately has she persisted in forming a fabulous notion of my character and acting on the false impressions she cherished. But, at last, I think she begins to know me: I don’t perceive the silly smiles and grimaces that provoked me at first; and the senseless incapability of discerning that I was in earnest when I gave her my opinion of her infatuation and herself."