Heathcliff is often compared to the devil because most of his attributes are very satanic in their nature, but even the devil can ignite ample amount sympathy in people, only if one looks at things from his perspective.
Having fallen from God’s grace he does not want to be understood and just want to be as depraved and malignant as the expectations that are attached with him. Similarly, in case of Heathcliff, he has suffered victimization at the hands of adoptive family and people in general, therefore, he does not wish to be understood and most of his behaviour is drive by spite and malice for them.
Heathcliff like Satan is a tormented soul because the ardent feelings he had for certain people were only reciprocated by negativity and hatred, therefore he establishes vengeance as the only goal in his entire life. Heathcliff was powerless and weak as a child, and was tormented greatly by Hindley Earnshaw but when he gained financial power he used that to make everyone suffer at his hands and even gains control of the entire Earnshaw estate.