Rather than "evil", Heathcliff is a product of his upbringing. He was neglected, so he became neglectful. He was abused, so he became abusive. He was set aside from the rest, so he cast everyone else aside from himself.He was treated unfairly, so he became scornful and angry.
Heathcliff's is the typical case of having a chip on your shoulder and hanging on to the complexes one earned in the past. He really was not evil, because he had reason enough to behave the way he did. He was dark, though, but it was a consequence of his life. All Heathcliff really is, is a wounded soul trying to get back at those who hurt him, and trying to find an indentity.