Binary Opposition!

Everything in the world has an opposite. These extremes can be placed on a scale and most characters would appear in between the extremes, rarely being completely one end of the scale.

Opposites are extremely useful in media for letting the viewer know whose side they're on. For instance they will easily discover who main enemies are as they are the opposite of the protagonist. There are often some examples that cause confusion. For instance in the Warcraft series, the Alliance and the Horde are at war. The humans are in the Alliance and we are therefore siding with them and automatically assume the Horde are evil, yet they are at a similar position in the Good-Evil scale.

One TV show that uses opposites as a large theme throughout is Twin Peaks.

In Twin Peaks there is an idea that your soul enters a place known as the Waiting Room. Your soul is then tested against multiple extremes of emotion. Depending on if you pass these tests, your soul either enters the White Lodge (Good) or the Black Lodge (Evil)

The Waiting Room - Twin Peaks

Other examples of opposites noticed throughout the series are

Laura Palmer's double life - She was loved by everyone, incredibly intelligent and popular in school. She was also a prostitute and addicted to cocaine.

Doppelgangers - Once your soul is trapped in the Black Lodge, an evil doppelganger of you escapes into the real world and replaces your original.

Dale Cooper/Windom Earle - While they were partners in the FBI, Windom becomes jealous of his partner's love for his wife, and attempts to murder them both. He then seeks out the Black Lodge to use its power for evil. Dale, however, remains an FBI agent and has a very good soul and seeks to stop Windom.