The study of serial killer collaboration has received little attention in academia. While current explanatory theories of serial homicide can include subtypes of serial killers that operate alone, the study of collaborating serial killers has been neglected. In this paper, an integrated, explanatory theory of serial killer collaboration is proposed. The theory builds on concepts from social learning theory, the trauma control model, and relational self theory and aims to examine what interpersonal dynamics that characterize the partnerships of collaborating serial killers. Five cases of collaborating serial killers have been analyzed and compared with focus on individual life histories and how these are reflected in the interpersonal dynamics in serial killer collaboration. The study found that serial killer collaboration is fundamentally characterized by a mutual need for human connection and approval, and that sociocultural role expectations affect the interpersonal dynamics of collaborating serial killers in terms of dominance, victim-preference, victim-acquisition, and method of murder.