By reframing behavioural radicalisation as predatory aggression, this chapter introduces an analytical framework applicable across diverse contexts—from domestic terrorism to ethnic strife. Unlike bargaining aggression, which seeks negotiated gains, or reactive aggression, which is defensive, predatory aggression is driven by the intrinsic aim of eliminating or depowering perceived adversaries. This perspective clarifies why radicalised individuals are often viewed as ‘psychotic’, linking this perception to a psychological inversion of costs and benefits where harm to others becomes personally rewarding. By identifying envy and hatred as central emotions, the chapter advances a functional approach to the study of radicalisation. Through its application to ethnic violence, this perspective illuminates how predatory aggression shapes collective conflicts, offering a more precise conceptual boundary for behavioural radicalisation across different contexts.