It's OK if you hate Grandcourt. We don't like him at all, either – that just goes to show how well George Eliot created this villain. Everything about him is mean, and he just oozes with sleaze. Grandcourt seems fine enough when we first meet him, but pretty soon it becomes clear that he is a selfish jerk who doesn't care about anyone but himself and his own gain. He just swoops into town one day to check out Diplow, since he's pretty confident that he'll inherit the house from Sir Hugo. He doesn't think for a second about how he might displace Lady Mallinger and her daughters. It seems almost permissible that he does that, though, because that's how society worked in those days – it wasn't really anything personal. Men inherited the property, and women relied on their kindness. Fine.
But let's think about how little regard Grandcourt has for the people who are actually close to him. Lush, for example, has worked for Grandcourt for the last fifteen years – you'd think that Grandcourt would give a hoot about him by now, but no. The narrator repeatedly compares Lush to a puppy that has been kicked. Come to think of it, Grandcourt totally seems like the kind of guy who would kick a puppy. But we digress. Aside from Lush, think of Lydia Glasher. She's the mother of his four children, and we learn that when he first got with her, Grandcourt was more in love than he ever thought he could be. Just as easily, though, Grandcourt falls out of love with her. She's ruined; Grandcourt destroyed her marriage, and now she can never re-enter society and live the same way she did before.
Of course, it's through Grandcourt's relationship with Gwendolen that we learn the most about the guy. He plays games with her from start to finish. He knows that Lush arranged for Lydia to tell Gwendolen everything about their relationship. Still, Grandcourt keeps his lips zipped and sits back and watches Gwendolen squirm with self-reproach and unhappiness. He loves wielding power over her. She has to do exactly what he likes, and she has no control in their relationship. No wonder she isn't all that sad about his death itself – she worries more about the fact that she's a murderer. She couldn't care less if she never sees him again. You know what? We'd probably feel the same way.