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.