This paper proposes a theoretical framework for understanding fat stigma and its impact on people's well-being. It argues that stigma should never be used as a tool to achieve public health ends. Drawing on Bruce Link and Jo Phelan's 2001 conceptualization of stigma as well as the works of Hilde Lindemann, Paul Benson, and Margaret Urban Walker on identity, positionality, and agency, this paper clarifies the mechanisms by which stigmatizing, oppressive conceptions of overweight and obesity damage identities and diminish moral agency, arguing that the use of obesity-related stigma for public health ends violates the bioethics principles of nonmaleficence, autonomy, and justice.