The Invisible Man almost seems like a bad sitcom's version of a teenager, moping around about how no one understands him and everyone's out to get him. We can agree that he makes his own life harder by being a jerk to people, but we at least have to admit: everyone is out to get him. This is especially true at the end of the story, when the whole countryside is organized to find and kill him. So, if the Invisible Man is the protagonist, then the antagonist could be, well, everyone else. After all, they're the ones who make his life harder and prevent him from reaching his goal (by, you know, kicking him to death).
Yeah, we know. He can't be the protagonist and the antagonist. Or can he? Sure, everyone in England is out to get the Invisible Man, but we have to remember: his plans aren't for everyone to get the perfect little puppy. Not even close, actually. Instead, his plans are to murder his way to the top and rule like a crazy tyrant. So yeah, even if he's the main character of the book, we might want to think of him more as an antagonist, making everyone else's life difficult (or, in the case of Mr. Wicksteed, making his life…non-existent).