Mr. Monster

Second book in the "John Cleaver" trilogy by Dan Wells, following I Am Not a Serial Killer and followed by I Don't Want to Kill You

John Wayne Cleaver has always known he has a dark side, but he’s fought hard to repress it and live a normal life — separating John from Mr Monster to survive. But after unleashing his dark side and destroying the vicious killer that was terrorizing his town, John's inner monster is getting stronger and harder to contain.

And now more bodies are being discovered...

As the police fail to catch Clayton County’s second serial killer, John is going to have to use his secret knowledge of the first demon-killer to trap the second...but will he be able to avoid suspicion falling on him, and, in the face of extreme horrors, will he be able to restrain Mr Monster?

This book has examples of:

 * Ancient Conspiracy -
 * Bound and Gagged
 * Boy Meets Girl - John and Brooke.
 * A Chat With Satan - John and the demon.
 * Cliff Hanger -
 * Drunk On the Dark Side - John uses this as a Batman Gambit against the demon, who can sense whatever he feels. He's just barely able to put it aside, and it still ruins one of his relationships.
 * Electric Torture - There's a scene in the book devoted almost entirely to this.
 * Emotion Eater -
 * Heroic Willpower - John is a sociopath, but he's also the protagonist. How? A whole lotta this trope.
 * I Did What I Had to Do - A big focus of this book is the fallout for John's psyche after he did what he had to do in the first book.
 * I Have Your Wife - See also tropes such as An Offer You Can't Refuse or Interrogation By Vandalism.
 * Kick the Dog - Or, rather, set fire to a cat
 * Pyromaniac - John goes out and sets fires in order to let off steam when he's feeling pressure.
 * Reluctant Psycho - John keeps himself 'normal' by not crossing certain lines. But sometimes you have to cross one of them in order to not cross another...
 * Ship Tease - John and Brooke's dates never end well.
 * Shut UP Hannibal
 * The Power of Love - John gives the demon a speech on this near the end, which is odd, coming from him.
 * They Look Like Us Now
 * This Is Your Brain On Evil - John's entire struggle in life is with this darned trope.
 * The demon in this story, too. Only this one has given up struggling and is enjoying every minute of it. Given how psychological the book is, the parallels shouldn't be surprising.
 * Too Clever By Half - John knows things about serial killers that even Detective Foreman doesn't know.