This is a difficult issue in this novel, because I believe that Bronte tantalises us by giving us enough evidence to suggest that Heathcliff's madness in later life is due to the cruelty he endured whilst a child, but at the same time she also gives us enough proof to show the opposite, that his madness is something that he was born with and is an innate part of him.
However, if I was trying to prove your statement, I would look at such quotes as the following, which comes from Chapter 4 and is Nelly's initial assessment of Heathcliff soon after he arrives to the Earnshaw household:
He seemed a sullen, patient child; hardened, perhpas, to ill-treatment: he would stand Hindley's blows without winking or shedding a tear, and my pinches moved him only to draw in a breath and open his eyes, as if he had hurt himself by accident, and nobody was to blame.
Also note the way in which Hindley mistreats Heathcliff almost as soon as he returns to Wuthering Heights after his father's death:
He drove him from their company to the servants, deprived him of the instructions of the curate, and insisted that he should labour out of doors instead; compelling him to do so as hard as any other lad on the farm.
Such consistent mistreatment and abuse suffered as a child could easily marr Heathcliff's character, thus causing his madness later on as an adult and consuming him with the desire to gain his revenge on Hindley.