There is a crucial turning point during the climax of “Bad Santa” that polarizes me as a viewer.

In a hasty, rather embarrassing moment, Billy Bob Thornton — playing the foul-mouthed, sexually perverse and all-around nasty title character — realizes he shouldn’t be so harsh. And then, a movie that has already been running low on gas loses more of its redeemable qualities.

This comes as a huge disappointment for two reasons: The film’s director, Terry Zwigoff, also made “Ghost World,” one of the most effective character studies of the last decade. It is also a letdown because, although the man is an absolute creep, Thornton is a phenomenal actor.

Thornton plays a department store Santa who works for little more than chicken feed alongside his fellow conman, played by Tony Cox, who rob the store on Christmas Eve. Bernie Mac plays a security officer who’s onto them — but rather than bust them, he wants a piece of the action.

The main focus of the story is Thornton’s relationship with a nerdish boy, played by Brett Kelly. Thornton ends up taking shelter under the kid’s roof, who lives with his grandmother while Dad’s off in prison. The story involving Thornton and the kid’s relationship is crucial to the ending, though nothing going on in “Bad Santa” really matters.

“Ghost World” was a movie that unequivocally invested itself into its characters. The film never plays or manipulates the audience. “Bad Santa” does the opposite: It takes its one-trick pony of an idea and plays it to death. Characters lack motivation, aside from pure greed, and it spends far too much time eavesdropping on characters no one should care about.

It’s a shame to see Thornton wasted. He’s a character actor who usually seems to make use of every one of his scenes. Even in awful movies like “Waking Up in Reno,” he manages to spark some kind of awkward likeability out of his character. He doesn’t do this in “Bad Santa.” The film’s tagline is “He doesn’t care if you’re naughty or nice.” If he doesn’t care about anything, why should we?

Depending on your sense of humor or the mood you’re in, it may be very humorous to listen to the drunken Thornton tell kids to fuck off and spew other vulgarities. To give it credit, the film is extremely hilarious during the first act. But unfortunately, the joke wears thin as the runtime marches on.

“Bad Santa” had the term “holiday classic” in its grasp. But, like another recent holiday bomb named “Love Actually” (also with Thornton), it fumbled its uncomfortable mood transitions and underdeveloped characters. This movie isn’t as guilty of the charge as “Love Actually,” but it did — at least for a while — interest me in its story.