How do you take a good character and make them evil? And why would you want to do this? Brandon, Dan, Mary, and Howard answer that second question first, and then walk you through the process of doing this.
Come up with a list of three things that are important to your main character. Push one of those things out of alignment so that it will draw your character to the antagonist’s side.
Hard Magic, by Larry Correia, narrated by Bronson Pinchot