If a film has a protagonist, there must, by definition be an antagonist. The protagonist attempts to reach their goal but is confronted with every possible obstacle. Those obstacles, are created by, or even are the antagonist. In other words, the antagonist does not always have to be a character in your story. It can be the environment, society, an energy, animal or even a inner struggle within the protagonist themselves. In Danny Boyle's film 127 Hours a mountain climber becomes trapped under a boulder while canyoneering alone and resorts to amputation of his own arm to free himself. Here, at first glimpse it would appear that the boulder is the antagonist, but later, as the story unfolds, there is an inner struggle in the character. His own fear of pain becomes an obstacle to freeing himself, and thus an antagonist.
It is often stereotyped that the antagonist is evil and makes morally reprhensible choices. This does not have to be so, and in fact the antagonist might be the good guy in your story. In Breaking Bad, the Protagonist, Walter White who chooses a life of crime, has a rather boisterous gung-ho brother-in-law, Hank Schrader, who works for the DEA. Hank's job is to catch the criminals behind a new type of amphetamine hitting the streets called "Blue Meth". Hank is unaware the his brother in law, Walter White is one of "the cooks" of this Blue Meth. Here, the antagonist in fact holds the moral upper hand. It's a rather masterful stroke by the creator of this show, Vince Gilligan, to not only make the antagonist morally a better person, but also to connect the protagonist and antagonist so closely, by making them related by marriage and seeing them interact at family gatherings. It certainly adds more obstacles and tension in the path of the protagonist. In deed, if you are going to make your antagonist a character, think of ways to connect them with the protagonist. One final word about Breaking bad, Vince Gilligan's intention was to make a show where the protagonist slowly transforms into the antagonist, so in fact Walter White ends up becoming the antagonist!
Do we always hate the antagonist? Well no, but it helps. If we care enough about the protagonist to be bothered to see what happens next , then naturally we will dislike the opposing forces. We may even fear, be in awe, be perplexed, or be overwhelmed by them.
In my opinion, create a sense of uncertainty in your story (for film), and never let the audience know too easily where the journey takes them, and they may even be bothered to follow that journey. The antagonist, I think, plays the most important role in creating that uncertainty.
