There is as yet no widely agreed-upon solution to the standard textbook problem whether actively shutting off a life-sustaining medical device, e.g. a respirator, and thus bringing about a patient's death amounts to active killing or just to an omission of further treatment. Apart from a range of astutely contrived case examples and respective particular solutions proposed in the literature, there seems to be no consensus on the normative principles such solutions should be grounded in, not even on the need for such principles beyond sheer intuition. The present paper attempts to develop a normative approach based on fundamental principles of law. From this perspective, what is decisive for the question of ‘killing or letting die’ in such cases is not that death ensues from a behaviour that is active and relevantly causative, but rather, whether or not the agent in performing the deadly act transgresses the boundaries of the domain of his or her sole normative authority, and thereby intervenes in the protected sphere of another. Unless he or she does so, their behaviour cannot be classified as active commission regardless of the amount of causal activity it may display and regardless of its potentially harmful consequences. This conception is spelled out in detail and tested in a range of case examples, as are several of its corollaries that deviate from standard type solutions.