How do you make a good bad guy?

Have you ever read a novel or watched a movie and found yourself wondering what the bad guy does when they are not trying to take over the world or kill the protagonist?

Too many antagonists start out their lives with a goal of evil, and that is not how it should be. The antagonist should start their life wanting to do something just because they are interested in it, then have the world around them warp them.

Look at Sweeney Todd. He was warped by the world from a loving husband and father into a focused maniac that is beheading people in his barber shop. He did not start out as a killer, the events of his life brought the killer inside of him out.

Did the school bully beat up the son of your protagonist because they wanted to steal lunch money, or was the lunch money an excuse for beating the smaller kid up to let the bully vent their anger at being beat up by their father? Or had someone else beat the bully up several years before so now the bully is beating up the smaller kids?

Did your antagonist spring into life wanting to destroy the protagonist, or did something happen to make them hate the protagonist? Look at the Spiderman movies. The shift from friends to enemies between Peter and his friend is a great example on events causing bad guys to be created. It was the event of seeing his father killed that created Spiderman's enemy and left Peter Parker balancing a delicate line between being a best friend and knowing that he was also the man his friend hated the most in the world.

Look at your antagonist when you are creating them and ask yourself what happened in their lifetime that they are how they are. Did they have a drunk and abusive guardian growing up? Did something bad that happened to one of their friends, or a family member, and they blame a specific character for it, rather the character was really to blame or not? Were they witness to something horrific that left them with a warped sense of right and wrong?

Do not just create a character that hates the protagonist because they think the protagonist killed their father - that's been done - create something memorable. Take some time to think about a unique reason for what made the antagonist hate the protagonist and then examine that idea for flaws. Has it been done before? How often? If it has only been done a few times you might get away with it, but if it has become a cliché you are better off finding a different variation on it. Don't just make your bad guy memorable - make his reason for being bad memorable.