There HAS to be an underlying psycological cause
Conclusion to the Psychological Effects of Combat - Dave Grossman, Author It is often said that "All's fair in love and war," and this expression provides a valuable insight into the human psyche, since these twin, taboo fields of sexuality and aggression represent the two realms in which most individuals will consistently deceive both themselves and others. Our psychological and societal inability to confront the truth about the effects of combat is the foundation for the cultural conspiracy of repression, a deception and denial that has helped to perpetuate and propagate war throughout recorded history.
In the field of developmental psychology, a mature adult is sometimes defined as someone who has attained a degree of insight and self-control in the two areas of sexuality and aggression. This is also a useful definition of maturity in civilizations. Thus, two important and reassuring trends in recent years have been the development of the science of human sexuality, which has been termed "sexology," and a parallel development of the science of human aggression, which D. Grossman has termed "killology." There is a universal consensus that continued research in this previously taboo realm of human aggression is vital to the future development, and perhaps to the very existence, of our civilization.