The paper aims to define the psychological foundations of lone wolf activism. The analysis that facilitated attaining this goal was based on the Nietzschean concept of resentment theory of compensatory revaluating values that explain the relationships between inferiority and fundamentalism, fanaticism, and ideologically motivated violence. Based on a phenomenological examination of the phenomenon, the author demonstrates that lone wolf activism is founded on two psychologically and sociologically determined successive processes. The first one occurs when a sense of personal inferiority becomes the source of an envy-based hostile attitude toward the world. Later on, this feeling, due to personality defence mechanisms, which bring about the falsification of “primary desires” and the generation of “secondary desires”, transforms into fundamentalism. The second process takes place when, as the result of fundamentalist legitimisation that arise on the level of social rivalry, given fundamentalism is destituted, resulting in fanaticism. The author believes that the knowledge of both processes is necessary to recognise and combat the terrorist activity of lone ideologically motivated individuals.