Why They Kill by Richard Rhodes is a study of crime and why criminals commit violent acts. Specifically, it is a critique of the life and work of criminologist Lonnie Athens. Athens is a criminology professor at Seton Hall University whose work Rhodes encountered almost accidentally. Rhodes's work is a re-examination of Athen’s 1992 monograph The Creation of Dangerous Violent Criminals.

Athens's thesis is that criminals are acculturated into violence by means of violence perpetrated against them. Criminals, according to Athens (and Rhodes, following Athens), are created by society. The purpose of the book is to examine Athens's theory as it applies to modern criminals.

The themes of the book include violence, crime, and family. According to Rhodes, crimes are committed by individuals who have been abused. Both Rhodes and Athens suffered through abusive childhoods. Violent acts are themselves manifestations of the outrage that individuals harbor against the community, which is internalized as the principle offender. Some of the perpetrators studied by Rhodes include Mike Tyson (a rapist) and Cheryl Crane (who stabbed her mother’s boyfriend).

Violence is one stage in a four-stage process that represents the culmination of the brutalization and belligerency processes (themselves marked by the individual being offended against by someone close to them and then emotionally equipping themselves for violent behaviors, respectively).

Rhodes includes details of both his and Lonnie Athens's childhoods. The former was abused physically and psychologically by his stepmother and the latter by his aggressive father, Pete. Pete was a blue-collar worker whose ethnic identity as a Greek made him socially marginalized, exacerbating his already aggressive tendencies. In both cases, intervention by other family members and social support services prevented the propagation of violence. Therefore, Rhodes's book is in large part a call to action to advocate for the establishment of these support services in schools and community centers where young adults can access them.