I would bet that the marginal cost of that movie is actually higher than the cents it costs to produce that bag of M&M's. Even if the marginal cost of movies is 0 the cost of a bag of M&M is so low it might as well be 0. Does that now justify stealing the bag of M&M? No.

That does not even matter since looking at marginal cost of a movie makes no sense. Do you want to see movies? If yes then pay for them, if no then simply don't watch them. Justifying stealing isn't the way to go.