The complexity and diversity of unlawful killings — particularly those involving mentally impaired offenders — does not fit neatly across the binary distinction of offence/defence that structures criminal law. This is demonstrated in the Australian State of Victoria, where cognitively impaired homicide offenders who fail to meet the strict remit of the mental impairment defence have no (partial) defence or offence available to them which adequately captures their levels of criminal responsibility, moral agency and culpability. This makes the sentencing of such offenders not only particularly complex but means that the only stage in which both moral and legal culpability can be considered is in mitigation. This article argues that a progressive framework is needed to permit a small minority of (mentally impaired) homicide offenders to be simultaneously inculpated and (partially) exculpated. Accordingly, we propose introducing a model of diminished culpability manslaughter in Victoria, drawing from Loughnan’s seminal reconceptualisation of ‘diminished responsibility manslaughter’ as an offence-cum-defence, which renders the diminished accused differently liable. Informed by a study of all homicide cases (n=647) sentenced in Victoria between 1 January 2000 and 31 July 2017, we argue that this model would not revoke legal capacity and would instead enhance the legitimacy and coherence of criminal law procedures, allowing a wider range of more legitimate convictions and reflective sentencing dispositions.