What is the theme in Plum Island by Nelson DeMille?

Revenge is a central theme in the book. John Corey is a violent man who views the world with a jaded eye. He is accustomed to observing scenes of violent death, but when the victims are people he has recently met, he feels a special obligation to seek out their killer. When his newfound girlfriend, Emma, is raped and murdered by the same person who killed the Gordons, John is driven to a white hot hatred and drive to not only find the killer, but to exact a lasting revenge upon him.