Arthur Fleck decides that some absolutely terrible state is who Arthur Fleck is authentically and becomes the Joker.
Mental illnesses are not per se associated with guilt. Most mentally ill individuals today know that mental illness is not their fault. Individuals who are mentally ill can feel intense guilt but that would be due to the committing of some terrible deed. The terrible deed not mental illness per se would engender the guilt. Shame is not per se associated with mental illness. Shame arises when one loses face before other individuals. All too frequently individuals with mental illness handle situations with other individuals poorly but the shame does not arise per se from being mentally ill.
A state, however, that is closely associated with mental illness is inauthenticity. Individuals with mental illnesses can hold they are not leading authentic lives. Paradoxically mental health professionals should not push individuals with mental illnesses to a point where mentally ill individuals are willing to state they are living authentically. To accept some terrible state as an authentic state is a prelude to disaster. And if one is in some terrible state one must accept that there are a ways to go, perhaps a long ways to go before one is living authentically. As treatments for mental illnesses are very frequently mediocre mental health professionals appropriately mainly provide support rather than treatment. Authenticity is a goal but holding what must be inauthentic as authentic is not even a half-way decent answer. A goal of patients/clients reaching authenticity has been a disaster in the field of mental health.
For individuals to hold that terrible states are authentic state for such individuals is all too frequently the prelude to absolutely horrendous deeds. Before every mass shooting the shooter says ‘I hate these random individuals, I hate the world and hate who I am and that is who I am authentically. There is no downside to this act in terms of who I am authentically.’