"The nuisance of her presence (Isabella's) outweighs the gratification to be derived from tormenting her!" -- Heathcliff (pg.188)
After marrying Isabella, Heathcliff wastes no time in telling her that he doesn't actually care for her, he just found their marriage to be convenient. His motivation for doing it was the knowledge that it would upset Edgar above all else, and he knew it would allow him to start conniving his way into attaining Thrushcross Grange for himself. If Isabella outlived Edgar, which was likely at the time, the Grange would become her property and therefore in his control. Throughout Wuthering Heights, I find most of Heathcliff's motivations for his actions to be rather interesting. It's almost as if he wants everyone around him to be more miserable than he is because of the overwhelming grief he feels for Cathy's demise. Though I sympathize with him to an extent, for the most part I find his actions to be unpredictable and overly cruel. I keep thinking that at some point in this novel he is going to change dynamically and become a benefactor, but so far I have been entirely incorrect in that conjecture. He truly is one of the coldest characters I've come to know.