Flood by Andrew Vachss
The main character a private detective known as Burke, comes to the assistance of a young woman named Flood. She is searching for a psychotic freak known as the Cobra, a child molester who has murdered her closest friend's child. After much fighting and chasing her down Burke finally accepts the job. Not necessarily a hitman, Burke is more of an avenging angel for hire, if he can be convinced the cause is worthy, and his dog Pansy doesn't rip a prospective client to shreds. His existence consists of an odd assortment of hookers, restaurant owners, gambling, and Max the Silent, his spiritual brother, and possibly the most dangerous man on Earth.
This is not a quick or easy or clean read. It makes you stop and think really shudder a bit as the story revolves around an underworld of S&M, perversion, and snuff films. Unfortunately we know from watching the news these things really do exist. Vachss is a lawyer specializing in child abuse cases and bases his stories on many true life situations he had found himself involved in.
The characters are interesting and intriguing and I'll probably read more but it will be awhile before my brain can really take another one.