The characters in the novel have a great capacity for violence and hatred, and revenge is one of Heathcliff's greatest motivations. Is Heathcliff justified in the revenge he takes on Hindley? Do you think Heathcliff's childhood experiences at Wuthering Heights have contributed to his monstrous behavior as an adult?
You need to re-read the earlier sections of the novel that describe the character of Heathcliff before he left Wuthering Heights and before he really starts being abused and mistreated by Hindley. Obviously your question taps into a whole nature/nurture issue which has been brilliantly explored in the somewhat disturbing novel We Need to Talk About Kevin, but Nelly Dean certainly suggests that Heathcliff already had the potential to be a "difficult" child well before Hindley truly got his hands on him. His abuse certainly didn´t help, but, as pippin1313 suggests, Heathcliff is an amoral character - someone to whom normal values don´t relate. I also agree that gaining revenge through abusing Hareton, Hindley´s son, in turn, is not justified. If it was just about Hindley, then it would be a different issue, but this introduces a disturbing element of repetition and how violence is passed on down through the generations in this novel.
Morally, there is no justification for taking revenge. Heathcliff is almost amoral and therefore has no issue with the taking of revenge on others. His revenge involves so many more people than just those he feels have wronged him and this is what makes him abhorrent to the reader at times. His revenge on Hindley may be justified to an extent but the fact that he uses Hareton to exact it is totally unjustified. Heathcliff also takes his revenge to the ultimate degree, so that rather than just payback, it becomes the total debasement of Hindley.
Heathcliff's experiences as a child totally contributed to his behaviour as an adult, although his origins seem to give him a propensity towards violent behaviour. It raises the question of nature versus nurture. In this case, it seems that it is a bit of both.