A sympathetic antagonist has several traits that a writer must consider. He must be believable and adapt to changes, whether he wants to or not. His flaws come from a survival instinct and the fear of discovery. It's the consequences of discovery that motivates his actions. His secrets should be revealed gradually throughout the novel.
He doesn't necessarily need to dislike the people he hurts and he only kills when threatened. This does not apply to serial killers, but my antagonist isn't mentally unbalanced. A believable antagonist acts on his desires and is highly motivated by something only the writer knows. He must have a few sympathetic traits, such as loving animals. Birdman of Alcatraz fits into that category. Robert Franklin Stroud was convicted of manslaughter, but his love of birds helped him pass the time in prison.
A few novels with the antagonist as the main character are, Phantom of the Opera by Gaston Leroux, the musical, Wicked, by Winnie Holzman, and The Murder of Roger Ackroyd by Agatha Christie. All of these characters have a few good traits and some manage to mend their evil ways.
When I was a child, one of my favourite characters was the Grinch, created by Dr. Seuss.Cold and Heartless, the Grinch doesn't want anyone to be happy and spends his time sabotaging the sunny community of Whoville.
Using the antagonist as the main character opens a whole range of opportunities for an author. Just remember that he must strike a cord of sympathy with the reader and become almost an anti-hero.
