Heathcliff is one of the most villainous characters in all of literature. He is referred to as a devil over and over again throughout Wuthering Heights.
When he is first adopted by the Earnshaws, Mr. Earnshaw notices that "it’s as dark almost as if it came from the devil’" (IV). Despite his rough childhood, Heathcliff seeks Nelly out to help him become more attractive to Catherine. She responds by noting his "thick brows" and his eyes like "black fiends" and "devil's spies" (VII). She seems to be saying that he will never be able to mask his devilish appearance.
Later, his personality becomes evil as well. His wife, Isabella, finds out soon after her wedding that Heathcliff is more than unkind. She asks "Is Mr. Heathcliff a man? If so, is he mad? And if not, is he a devil?" and later notes that no matter when it happened, his "... devilish nature would have revealed its existence somehow" (XIII)
Heathcliff even frightens his son, claiming to be "the devil himself" to scare his son so much in front of Cathy.
Ultimately, Heathcliff forces Cathy to marry his son or perhaps never see her ailing father again. Nelly shows her rage when she asserts that she would fetch Cathy home even "... if that devil be killed on his own doorstones in trying to prevent it!(XVII).
The final time Heathcliff is refered to as the devil comes from Cathy. After he has taken everything from her and made her a prisoner, she is still able to recognize "You are miserable, are you not? Lonely, like the devil, and envious like him? Nobody loves you—nobody will cry for you when you die! I wouldn’t be you!’ "(SVIII).
Heathcliff lets his passions get the better of him and dies lonely and sad, if not a devil.