This paper considers the ethical challenges facing the development of robotic systems that deploy violent and lethal force against humans. While the use of violent and lethal force is not usually acceptable for humans or robots, police officers are authorized by the state to use violent and lethal force in certain circumstances in order to keep the peace and protect individuals and the community from an immediate threat. With the increased interest in developing and deploying robots for law enforcement tasks, including robots armed with weapons, the question arises as to how to design human-robot interactions (HRIs) in which violent and lethal force might be among the actions taken by the robot, or whether to preclude such actions altogether. This is what I call the "deadly design problem" for HRI. While it might be possible to design a system to recognize various gestures, such as "Hands up, don't shoot!," there are many more challenging and subtle aspects to the problem of implementing existing legal guidelines for the use of force in law enforcement robots. After examining the key legal and technical challenges of designing interactions involving violence, this paper concludes with some reflections on the ethics of HRI design raised by automating the use of force in policing. In light of the serious challenges in automating violence, it calls upon HRI researchers to adopt a moratorium on designing any robotic systems that deploy violent and lethal force against humans, and to consider ethical codes and laws to prohibit such systems in the future.