“Is Mr. Heathcliff a man? If so is he mad? And if not is he a devil?” That's what Isabella asked Nelly in a letter she had written to her in despair short after her and Heathcliff’s hasty marriage.
“I shan't tell my reasons for making this inquiry; but I beseech you to explain, if you can, what I have married.” She also writes that Heathcliff accuses Edgar for causing Catherine's illness and therefore will punish her; Isabella, in his place till he can get hold of Edgar. Yes, Mr. Heathcliff is a man; a man who due to his tragic childhood and great love for Catherine developed a very vengeful, cruel and manipulative behavior which sometimes even can be interpreted as madness.
Mr. Heathcliff was from the beginning an orphan brought to Wuthering Heights by Mr. Earnshaw. Hindley, Mr. Earnshaw's, son and Nelly hated Heathcliff from the day he arrived and treated him very bad. This was the beginning of a childhood full of tragic memories that would make Heathcliff very vengeful later in life. “He seemed a sullen, patient child; hardened, perhaps, to ill-treatment: he would stand Hindley's blows without winking or shedding a tear, and my pinches moved him only to draw in a breath and open his eyes, as if he had hurt himself by accident and nobody was to blame.” This quote shows how Heathcliff, despite Hindley's and Nelly's bullying, never tried to get back at them. Perhaps his rage has been building up inside him and then one day he decided to get his revenge
Also, an explanation for Heathcliff's cruel behavior is his great love for Catherine which is the only thing he cares for. Their love is very deep and spiritual but complicated and they have trouble expressing their love for each other and often end up fighting. The fact that they can't be together drives Heathcliff mad. His love for Catherine is his obsession, nothing else matters and he's willing to do whatever it takes to show his devotion even though it requires him to become a dark and