This dissertation examines how violent fantasizing influences the behavior of a brutal sub-class of murderers--mass and serial killers. Specifically, fantasy gives the perpetrator a profane catharsis due to his or her inability to cope with reality. The researcher identified, four common fantasy scripts: (Revenge Fantasy; Sexual, Sadistic and Misogynistic Fantasy; Suicidal-Homicidal Ideation; and Search for Validation through Infamy and Media Attention Fantasy) that more or less, play into the motivations and actions of mass and serial killers. Thus, it is important to understand why and how the killer moves from an all-consuming imaginative space to actually harming others. The methodology used for this research was "ethnographic content analysis" and, to a lesser extent, empirical phenomenology and semiotics. Source materials that were analyzed included: artifacts generated by the offenders prior to commission of their crimes (e.g., diaries, manifestos, blogs, drawings, photographs, and videotapes); official findings of governmental review panels; other public documents; survivor, witness or family accounts; news reports; and work conducted previously by other academics. This dissertation is particularly novel, in that the role of fantasy has not received much critical analysis with respect to mass murder. Likewise, the researcher's examination of current theory on the ontogenesis of moral dysfunction led to an original interpretation in the works of criminologists, Eric Hickey and Lonnie Athens. From a synthesis of Hickey's trauma-control theory and Athens' esoteric constructs of "self" and "other" a more cohesive understanding of the homicidal personality emerged. Essentially, the researcher argues that the intersection of early derailing influences and pervasive life losses result in a fragmented concept of self, which the now deeply unstable individual seeks to validate through violent fantasy and homicidal acts. It is further proposed that these findings may lead to future inquiry into: methods for early intervention and diversion of an at-risk population; and where the foregoing is impractical, better methods of detecting, mitigating the harm caused by and quickly apprehending these particularly violent offenders.