I don't think Heathcliff is a fiend. He's not crazy, even if he does act unstable. He is stereotyped by most. I agree with accessteacher that it is hard to draw specific conclusions, but for some reason I always felt sorry for him. He seems like a victim.
I think it is important to be aware of the very ambiguous position that Heathcliff occupies in the novel. Many critics have studied the way in which he is presented in a way that prevents us from making any clear judgements about his character. Certainly, he does unforgiveable things, and is presented more as a beast in the way that he is described at times than as a human, such as when Nelly Dean describes how he reacted to the news of Catherine's death in Chapter 16:
He dashed his head against the knotted trunk; and, lifting up his eyes, howled, not like a man, but like a savage beast being goaded to death with knives and spears.
But note too, how in the same chapter, just a few paragraphs before, Nelly Dean seems to suggest that in spite of how he is presented, he is just a man that we ought to feel sympathy for when she observes how moved by anguish he is at his beloved's suffering:
"Poor wretch!" I thought; "you have a heart and nerves the same as your brother men! Why should you be anxious to conceal them?"
Note the way in which Bronte creates a careful ambiguity concerning Heathcliff. Whilst he certainly does villainous things, he is presented as a man whom we should pity at various points, and we are always aware of the way that he was mistreated as a child in his upbringing. We are left with a conundrum when we debate the precise nature of Heathcliff.