Why does Charles Dickens create bad characters in his novels?

Good storytelling takes the listeners (or readers) into the story, such that they identify with one or more of the characters central to that story. The technical name for that person is the "protagonist."*

If nothing of interest happens to the protagonist, the story (obviously) is pretty dull. I mean, would even a four year old pay attention to Little Red Riding Hood if she simply took her basket of goodies to Grandma and then went home? Thus, the key to an interesting story is to have the protagonist face some sort of challenge. The entity providing that challenge (which can be any number of things, even the mind within the protagonist!) is called the "antagonist."

Again, to assist the reader in getting into the story, it helps if the reader likes the protagonist more than the antagonist. Thus, it is typical for a storyteller to make the antagonist someone unlikeable. In other Words, a "bad character."

The magic of Dickens is his ability to create characters we are interested in, even if we don't like them. Scrooge, for instance -- we find him repulsive at first meeting; but, after we learn what events and choices led to his nastiness, we WANT him to be redeemed.

* Even more technically, a protagonist is simply the central person in the story, and can be someone the reader does NOT like. However, most storytellers make the protagonist a person the reader will identify with.