While psychiatry often has the task of averting danger to third parties, the legitimacy of this task has been controversially discussed and repeatedly questioned. From an ethical perspective, coercive measures or coercive treatment in the context of danger to others must always be well justified. In this article, we first describe how measures against a person’s will can be ethically justified. With reference to the principle of soft paternalism, it is generally assumed that coercive treatment of people who lack decision-making capacity is only justified if it is in the person’s best interests and is in line with their anticipatory or presumed will. We then examine to what extent coercive treatment of people with mental illness who endanger others can be justified on soft paternalistic grounds. We discuss different theories of well-being and propose a hybrid subjective-objective theory of well-being as a suitable standard for assessing the well-being of people with mental illness. A case study is used to discuss in which cases coercive treatment can be in the best interests of a person who endangers others and corresponds to their presumed will, and to clarify when this is not the case, for example when the person’s well-being and will contradict each other.