One of the things you realize as you read this novella is that since it's about a man wrestling with his own inner demons, that man happens to be both the protagonist and the antagonist of the story at the same time. Dun dun dun!
Think about it—whose corner are we readers in? Whose transformational arc are we invested in? When we want to see a happy ending, whose eventual happiness are we rooting for? That's right, it's Scrooge. We really, really want to see that horrible old man change his ways because there is simply no other character in the text that we have nearly as much contact with.
But, at the same time, who's the big bad guy here? Who is the one stopping Scrooge from doing what he needs to and getting what he wants? Who needs to be removed in order for our protagonist to succeed? Yup, Scrooge again—or at least the hard outer shell that Scrooge has grown around himself in the second half of his life.