How do the themes of revenge and forgiveness develop the work of Wuthering Heights?

Heathcliff's devotion to Catherine, on the other hand, is ferocious, and when frustrated, he conceives a plan of revenge of enormous proportions. Catherine's brother Hindley shares her passionate nature, though he devotes most of his energies to degrading Heathcliff. In some respects the passion that Catherine and Heathcliff share is so pure that it approaches a kind of spirituality. "I cannot express it," says Catherine, "but surely you and everybody have a notion that there is, or should be an existence of yours beyond you." In the characters of Heathcliff and Hindley, who both feel slighted in love, Brontë draws a parallel between the need for love and the strength of revenge.