For Sartre, existential guilt is what we experience when we:

In Sartre's existential philosophy, existential guilt arises from the sense of failing to take full responsibility for one's own existence and choices. When individuals allow others to define them or make decisions on their behalf, they relinquish their freedom and authenticity. This abdication of personal responsibility leads to feelings of guilt because, in Sartre's view, each person is responsible not only for themselves but also for the impact of their choices on others. By permitting external forces to dictate one's identity and actions, a person neglects their own potential and the fundamental principles of existentialism, which emphasize personal freedom, choice, and authenticity.

In contrast, taking complete responsibility for actions, while essential to Sartre’s philosophy, does not generate existential guilt; rather, it affirms individual freedom. Staying isolated from societal influences may also suggest a rejection of external definitions, but it does not necessarily imply guilt. Feeling regret for past decisions is more aligned with an emotional response to specific actions rather than the broader existential guilt rooted in the concept of allowing others to shape one’s identity. Therefore, the essence of existential guilt for Sartre is connected to the experience of failing to assert one's own choices and values in the face of societal pressures.