Because there are multiple books, and a 2003 film, all titled "Monster," it is not certain that your question can be answered. Providing an author would be helpful.
If one peruses those books, both fiction and nonfiction, with that title, a common denominator is the presence of a psycho- or sociopath in a leading role. The use of the label "Monster" in describing a real or imagined character might be intended to convey what the author sees as pure evil, with little or no remorse behind the violent actions of the criminal. Such an individual carries out heinous crimes and is capable of taking life with no thought about the ramifications and with absolutely no pity for the victim or the victim's family.
The label "Monster," however, is also used with a sense of irony. Books written by or about somebody who has been labeled "monster" by victims, law enforcement, prosecutors, or the media, may be intended to demonstrate the dehumanizing manner in which the perpetrator is viewed by society. Remorse or a plea for understanding on the part of a convicted perpetrator who has been labeled as such is not unusual, especially in the case of alleged sociopaths who, by definition, have been shaped by their environment and would not otherwise have been predisposed to commit violent crimes.