kind of powerful, romantic love. It does get out of control in the case of these two lovers, and becomes evil.
Heathcliff is shown to be a dark and even hateful character the moment we meet him on the first page of the book. Lockwood sees himself as a kind of kin to Heathcliff, and sees the region as a fit for heir dark personalities:
A perfect misanthropist's Heaven---and Mr. Heathcliff and I are such a suitable pair to divide the desolation between us. A capital fellow! He little imagined how my heart warmed towards him when I beheld his black eyes withdraw so suspiciously under their brows (45).
Lockwood has no idea how much more dark and evil Heathcliff is. Heathcliff is shown to be an evil man even before he meets Catherine, so we cannot say that he learned to be evil because of his extreme love for Catherine. We see Heathcliff not as a loving person, not as a lover, but as a hater, a misanthrope, or a person who hates people. This might be a clue helping us to understand the evil part of Heathcliff's love. Perhaps a love for one person which grows out of a hatred of all other people will end up being evil, sooner or later. Perhaps true love leads a person to feel warm and loving toward all people, or most people. And perhaps a love which is evil is one which comes from a hatred for people in general and a feeling that one other person, the beloved, will heal the pain of that huge hatred. In Heathcliff's case, that did not happen. He was not healed by love, but driven even more mad than he was at the beginning of the book.
In contrast to Heathcliff, Catherine is pictured as a warm, playful and loving person in the early part of the book. However, the first important scene in which we hear of Catherine and Heathcliff together gives us a sense of the evil of Heathcliff and the possibility of Catherine being affected by that evil. Again, we must keep in mind that we do not have a single or simple definition of love...