Well my laptop is finally operational. I lost the old one though, for now, but I was able to save the hard drive. Ugh. Don’t you hate computer malfunction?
Okay, bad guys.
Your villain is half the story. Even if the reader doesn’t know who it is until the end, as in a mystery, he needs to be a worthy opponent for your protagonist. He needs to be able to match wits, strength, and psyches. He needs to be as stealth at being a bad guy as your protagonist is at being the good guy. It’s like a well choreographed dance, if done right, will keep the reader guessing until the very end who will win. Like Batman vs. the Joker- Batman is quick, crafty, and smart. The Joker is clever, ruthless and cunning. It’s the most basic good against evil fight.
As others have said, he has to be human. Multi-dimensional. He needs to have traits just like any other character even if he’s a serial killer. Remember this, even Hitler loved dogs.
He needs his own reasoning, no matter how warped, for doing the things he does. Give him a backstory. Was he abused? By who? How did he cope? How does that drive his actions today? Give him his own fears, interests, goals. What motivates him? What does he want out of this story? How will he get it? Make it personal. Because a psycho, just because, is boring.
Remember the movie Psycho? That worked so well because Norman Bates had an over-bearing, controlling mother, that he all at once hated and loved, and that shaped him from childhood. Everything she drilled into his head hit home and stuck with him, creating the twisted man with the sweet face who no one would suspect was a murderer.
And a lot of psychology goes into creating a good bad guy. Try reading up on the subject. Jung, Adler, Freud. You might be surprised by the inspiration and ideas that spring from delving into the human mind. Then, get into his head on the page. Try writing from the killer’s POV, if not for the story, then at least to better understand him. Dig around a little. Remember Hannibal Lector? He was a great villain. Watch Silence of the Lambs to see an incredibly scary, yet well fleshed out antagonist.