There is horror in carers turning into killers. Hospitals readily provide the means and opportunity. Doctors in the Japanese Imperial Army Biological Warfare Unit 731 killed experimentally, and in Nazi Germany to end Lebensunwertes Leben. Elsewhere motive is a puzzle. Beverley Allitt killed young children by injection or suffocation: Munchausen syndrome by proxy, perhaps, though murder with no intermediary. Benjamin Geen in Banbury engineered cardio-respiratory arrest in patients whom he might heroically resuscitate, if they were lucky. Colin Norris killed old ladies by injecting insulin, ostensibly because he disliked old ladies. Bad, not mad, maybe – but certainly dangerous to know.