How to Make Readers Like Creepy Characters

I said I'd give an occasional writing tip and here goes. Above is one of my all time favorite funny horror novels, Johannes Cabal the Necromancer, by Jonathon L. Howard. Johannes Cabal is one of the most awful protagonists you'll ever find, but he's so freaking likable in his awfulness. He's not kind, he's not good, he's not particularly honest, and has been run out of towns by crowds carrying pitchforks. In The Necromancer he has to convince 100 people to sell their souls to the devil so Cabal can get his own soul back. He has no scruples. He'll take anybody's soul with only his brother, Horst, keeping him  anything close to moral. "No children!"

Why do readers like this creep? Why do they like Sherlock Holmes? He isn't friendly, kind, loving, or nice, either. There are two ways to make an unlikable protagonist appeal to readers. (Okay, I think there is a third which is to make him or her funny, but that wouldn't apply to Holmes because he's not a barrel of laughs. Cabal is hilarious, though.)

The two ways writers make unpalatable characters appealing is A) Make them brilliant at what they do. If he/she isn't good, make them more outstanding than anyone else in their field. This applies to both Cabal and Holmes.  B) Make someone we like, fond of the protagonist. Holmes isn't likable, but Dr.Watson is. We like Dr. Watson and whatever he likes, we like.  Therefore, we like Holmes.

The same holds true for Johannes Cabal. He's funny and brilliant, but nasty. He's even nasty to his brother, Horst.  For some reason, Horst still has fraternal feelings toward Johannes even though Johannes did about the worst thing he could do to his brother. We don't just like Horst, we (at least in my case) adore Horst. He's smart, he's strong, he's moral, he's kind, he's handsome, and an outstanding dresser even though he's dead. Despite all this, he's still willing to help his rotten brother. If Horst likes Johannes, there must be something likable about him.

To sum it up, if you have an icky protagonist, make him or her brilliant and make an approachable character like him or her.