This thesis proposes the "Moral Plasticity Hypothesis" as a novel framework for understanding human nature's relationship to evil and cruelty. Rather than viewing humans as inherently good or evil, this work argues that human nature is characterized by evolved moral plasticity—an adaptive capacity for extreme behavioral flexibility that can manifest as either profound compassion or devastating cruelty depending on contextual triggers and moral foundation activation patterns. Through interdisciplinary analysis of philosophical, psychological, evolutionary, and neuroscientific evidence, this thesis demonstrates that evil is not a defect in human nature but an emergent property of our adaptive moral flexibility, with profound implications for understanding and preventing mass violence. The research synthesizes insights from moral foundations theory [1], evolutionary psychology [2], neuroscience [3], and historical case studies of genocide and mass violence [4]. The central argument challenges both traditional philosophical dichotomies of good versus evil and contemporary psychological theories that attribute cruelty primarily to dehumanization or individual pathology [5]. Instead, the Moral Plasticity Hypothesis posits that the same psychological mechanisms that enable extraordinary altruism and cooperation also enable extraordinary cruelty and violence, depending on contextual activation patterns. Key findings include: (1) Human moral intuitions are built upon evolved psychological foundations that can be activated in prosocial or antisocial directions; (2) The neural circuits underlying empathy and violence show significant overlap, creating a neurobiological basis for moral plasticity; (3) Institutional contexts serve as powerful amplifiers that can channel human moral plasticity toward either constructive or destructive outcomes; (4) Historical patterns of mass violence demonstrate consistent psychological mechanisms across different cultures and time periods, supporting the universality of moral plasticity. The thesis concludes with practical implications for violence prevention, moral education, and institutional design. Rather than attempting to eliminate "evil" individuals, effective prevention strategies must focus on understanding and modifying the contextual triggers that activate antisocial moral responses. This approach offers a more nuanced and scientifically grounded foundation for addressing humanity's capacity for both extraordinary good and devastating evil.