Abstract Most clinicians and mental health practitioners are reluctant to work with people with dangerous and severe personality disorders because they believe there is nothing that mental health services can offer. Dangerous and severe personality disorder also signals a diagnosis which is problematic morally. Moral philosophy has not found an adequate way of dealing with personality disorders. This paper explores the question: What makes a person morally responsible for his actions and what is a legitimate mitigating factor? How do psychiatric nurses working with this client group understand the awful things some clients do? What concepts do they need, if they are to know how to explain and how to react? It is suggested that dangerous and severe personality disorder is best regarded as a moral category, framed in terms of goodness, badness, obligation and other ethical concepts. It seems plausible that in important ways the dangerous and severe personality disordered client does not understand morality or understands it differently. The peculiar position of the dangerous and severe personality disordered individual in our system of moral responsibility stems from his apparent inability to see the importance of the interests of others. It might be more helpful to regard personality disordered clients as we do children: partially but not fully reasonable for their actions. We might regard the dangerous and severe personality disordered client responsible for those actions which he most clearly understands, such as causing others physical pain, but not for those with which he is only superficially engaged, such as causing emotional pain. The paper concludes by suggesting that the dangerous and severe personality disordered individual does not fit easily into any conventional moral category, be it criminal, patient, animal or child, and thus an assessment of his moral accountability must take into consideration his special circumstances.