This article examines the operation of the reformed English diminished responsibility plea in mercy killer cases. In particular, it makes three claims. First, it predicts that—like its predecessor—the revised doctrine will be stretched, where necessary, to accommodate these offenders. This is because (i) normative arguments remain for convicting them of manslaughter instead of murder and (ii) other partial defence routes will usually be unavailable. Secondly, it contends that such pragmatism will now be facilitated by a disconnect between (i) the defence’s post-reform narrowing and (ii) its ongoing interpretive flexibility. Thirdly, given that disconnect, it suggests that this pragmatism will be problematic. Notably, it will (i) compromise the plea’s newfound coherence and (ii) exacerbate unfair labelling of mercy killers. Ultimately, and more broadly, these difficulties reinforce recent calls for further homicide law reform.