Making an evil character takes a good understanding of how the human mind works. Being evil for the sake of being evil is not realistic, and generally not good reading. A good book, after all, has more to do with the human element and our place in the universe. This leads into the depths of good versus evil, and defining the two sides to something black and white. What about the gray area in between? What is your own concept of good and evil? Is this the norm of society, or do you want to make a point of what your view is on human nature.
A good ‘bad guy’ is a character of many parts. He (or she) should have reasons in doing what they do; causes for their actions. A bad guy can provoke fear into the hearts of those around him, or hate, or even pity depending on the reader’s own experiences. Some bad guys are a wonderful blend of their own chaos, taking the other side of the protagonist in the story. They serve to create the conflict.
An act of evil depends heavily on the reasons, the causes, and the actual events of the act. When someone murders another, ask yourself ‘was it out of desperation, anger, fear, or enjoyment’? Sometimes a bad guy simply has weaknesses. He might be mentally ill, or carrying an emotional burden. What you might consider evil might be understandable by some readers. For instance, murdering someone in self defense or by accident isn’t evil, but can spur the reader to hate the ‘bad guy’. We can hate him for his fears, cowardice, or sense of revenge as well.
Theending for the bad guy is generally a bad end. They ‘get their just desserts’, and the reader is left with a sense of justice in the world. Even more cliche’ is that the villain often does something evil to remind the reader that he or she is evil and therefore they deserve do die. Or do they? Wouldn’t a plot that allows the bad guy to find redemption be the better end? Do they perhaps escape to be evil another day? Is the reader left to wonder? Do you stay the course, or explore other avenues in plot design for your bad guy?
TIP: Put yourself in the place and time of the bad guy character of your story. Ask yourself about motive and fears. Why is this character on the opposing side of your protagonist? Write from his or her point of view, justifying their actions.
If you don’t have an established antagonist, create one. Provide a brief visual description, but also habits, fears, hopes, dreams, and passions. Make the person ‘real’.