In Wuthering Heights, what moral justification is there to defend Heathcliff inspite of his diabolical nature?
Well, I guess that depends primarily on whether you think that Heathcliff is a character that should be defended or not. This is rather a large question, and it seems to be related to the idea of whether Heathcliff is an essentially good character who has been made bad through years of suffering and ill treatment at the hands of various people such as Hindley, or whether Heathcliff is actually a thoroughly evil character who starts out evil and remains evil throughout the entire novel. This is a fascinating question, because I don't think that Bronte gives us enough proof to support either extreme. Certainly the novel focuses on the way that he is abused and mistreated as a child, but at the same time, his cold, calculated exploitation of old Mr. Earnshaw's preference for him is something that he uses to get back at Hindley whenever possible. Consider how he blackmails Hindley into exchanging horses with him:
You must exchange horses with me. I don't like mine; and if you won't I shall tell your father of the three thrashings you have given me this week, and show him my arm, which is black to the shoulder.
There is just enough evidence to support our view of Heathcliff as being both abused and both evil, and as we read through the novel, the majority of readers oscillate between these two perspectives of his character. I guess at the end of the day, Heathcliff is a human, and therefore he does deserve at least some kind of defence for his actions were he to be tried for them rather than outright conviction, but the character of Heathcliff presents the reader with a massive dilemma and is not a character that can be categorised easily. He remains throughout the novel and after we have finished it as a character who is tortured by his obsession and love for Catherine and who defies all easy answers about his actions and motives.