What are the motives behind Heathcliff's elopement with Isabella?
Heathcliff elopes with Isabella to hurt Edgar and to avenge himself against Catherine's marrying Edgar Linton, whom he detests. By marrying Isabella, Heathcliff also gains power over her and control of her money.
In Chapter 11, Catherine overhears Nellie's angered outcry against Heathcliff's actions outside the window as he has just embraced Isabella Linton. When Heathcliff enters the house, Catherine then asks him why he has not honored her request to leave her sister-in-law alone. Heathcliff counters by saying that Catherine has treated him "infernally" and that if she believes that he would not retaliate for this treatment, he will soon convince her of the contrary. Catherine then asks,
"I've treated you infernally—and you'll take revenge!....How have I treated you infernally?"
"I seek no revenge on you" replied Heathcliff less vehemently. "That's not the plan....If I imagined you really wished me to marry Isabella, I'd cut my throat!"
Later, it becomes apparent that Heathcliff has used Isabella as a means of exacting revenge against Edgar Linton, who has strictly forbidden his sister from having anything to do with Heathcliff. In addition, by marrying Isabella Linton, Heathcliff gains power over her and her money, and he can use both for his revenge.