My theory is that a truly nasty antagonist is essential for successful fiction. Indeed, he’s your most important fictional character.
Before I began focusing on the antagonist as the primary fictional character in any novel, novella, or short story, I dropped headlong into a huge pitfall. In several of my early novels, my antagonist wasn’t any major threat to the hero. Not only were those novels a challenge to write (because it was a struggle to create real conflict), sales were woeful.
Avoid that pitfall. When you’re creating a fictional character, focus on your bad guy. He needs to be truly mad, bad, and dangerous to know. Then make your protagonist someone who has no chance against this overwhelming enemy.