Callous‐Unemotional (CU) traits refer to a pattern of attitude and behaviors, including a lack of empathy, guilt feelings, and remorse, shallow or deficient affect, and a lack of concern about others' feelings and living, which are associated with severe antisocial behavior. Almost all studies on the association between CU traits and violence are performed at individual levels, necessitating the examination of CU traits in extremist large groups. In an attempt to present a theoretical framework on how CU traits develop and function in extremist large groups, the author performed deep reflections on historical records of brutally violent acts of totalitarian regimes and extremist large groups based on data from scientific articles, books, media sources, and newspapers. The author concluded that the violent acts of these groups towards their enemies meet the criteria of CU traits. Accordingly, the author proposed the term “Syndrome of Collective Callous‐Unemotional Traits (SCCUT),” referring to the emergence of a group of CU traits among members of an extremist group or mass of people. The symptoms of SCCUT traits are elucidated, followed by explaining how and under what circumstances emerges. Also, the implications of SCCUT for future studies and counterterrorism and public health policies are discussed.