See also: The Force, Philosophy
The Force has two aspects, one Light and one Dark. The Dark Side lurks in the shadows, whispering to Force-users, tempting them with quick and easy access to power. While seemingly stronger, The Dark Side is only easier. It consists of the destructive impulses of all living beings.
Anger, fear, hatred, and aggression are expressions of The Dark Side, and such emotions can quickly lead a Force-user down The Dark Side's corrupting path. Early on in their training, a Force-user finds that The Dark Side greatly enhances their abilities. After a time, The Dark Side demands more and more of those in its embrace.