Retaliation

One of the main themes in Wuthering Heights appears to be the motivations and consequences of revenge. Heathcliff, when he is young, is not treated as he would have preferred. He is made by Hindley into a person of much lower status than he could have been when he is put to work on Hindley's manor. He is also the person Catherine doesn't choose to marry, despite her unending love for him. He leaves when he overhears Catherine speaking of her engagement  because he thinks that he does not love her. When he returns, Heathcliff is out for revenge. His main goal is to get even with all those who have ever wronged him.
Fore example, Heathcliff gives money to Hindley in a seemingly eleemosynary manner. Hindley, however, is an alcoholic and Heathcliff knows that Hindley will spend all the money on alcohol and fall deeper into debt and the miserable life he has been leading after the death of his wife Frances. Eventually Hindley dies and Heathcliff inherits his manor, which Heathcliff sees as vindication. Because he feels so hurt by Catherine, he treats the young Catherine (her daughter) poorly, seemingly to seek revenge on the late Catherine by proxy of  her daughter.
In the long run, Heathcliff ends up inheriting two large manors in which he employs Catherine, her daughter, and others whom he treats quite cruelly. While Heathcliff lives a monetarily prosperous life in the end due to his shady schemes of manipulating love and people's emotions, he never really seems to live an emotionally fulfilled life. He ends up with no true love because he sought so much revenge on the woman whom he truly loved and who loved him. He dies with no real achievements to his name and no sign of true happiness to speak of.
The story of Heathcliff underscores that, while it may seem like the justified thing to do at the time, retaliation against those who you think have done something to upset you (which may not even be the case-i.e. Catherine) is always the wrong option. Revenge, and the hunger for vindication, fills a person with nothing but emptiness and cruelty, and no one can love someone who is cruel at heart.