After several incarnations of Wuthering Heights, courtesy of Emily Brontë, William Wyler, and Luis Buñuel, one can make the assertion that Heathcliff is indeed an ass. But the ways that he is represented in the three are slightly different enough to make someone take notice of changes.
In Emily Brontë's 1847 novel, Heathcliff was portrayed as merely a human wrecking ball. Everything and everyone he came in contact with was left in ruins. His main purpose is simply to get revenge on Catherine for smiting him so many years ago, and he did not care whom he hurt in the process. Using Isabella to hurt Catherine and Edgar was a prime example of this. He simply led Isabella to believe he loved her, but then he quickly pulled an about face and would have nothing to do with her once they were wed. He even told her that he could not stand the sight of her. All of this was designed simply to get back at someone whom he "loved."
William Wyler's 1939 film adaptation did one thing differently from the way it was done in the novel. It attempted to show a human side to the character of Heathcliff, perhaps even a softer side. It definitely showed such a side during the first part of the movie before he left, but also at parts during the movie when he was near Catherine. One could tell that, while so much of him wanted to hate her and have his revenge, there was still a part of his being that wanted only to love her, and so he was torn. Laurence Olivier did a magnificent job of portraying Heathcliff in Wyler's adaptation. His expression and movement during scenes really could help the audience to visualize his character emotionally being torn and miserable.
Luis Buñuel's 1954 adaptation of Wuthering Heights, Los Abismos de Pasion, was similar in some ways to Wyler's 1939 version. The attempt at showing a human side to the Heathcliff character was evidently attempted, but the audience was often left to supposing that Alejandro was once a kind, caring person. If one had not read Wuthering Heights, one would have only vague reference throughout the film to find out what was going on. The only reason that I cannot be totally sure of Alejandro's subtle breakdown is that the film was subtitled, so I could not tell any dramatic changes in tone of the voice of Jorge Mistral, the actor portraying Alejandro.
The novel and both film adaptations were well done; and as I said, slight differences were made that can dramatically change the audience's interpretation of a character. In the film adaptations, Heathcliff seemed to be given something that he was definitely missing in the novel--a soul.