Humans have a long evolutionary history of violence. The psychological mechanisms underlying aggression can be viewed as “solutions” (albeit undesirable solutions) to any one of a number of adaptive problems that exist in social life. Sometimes that aggression takes the form of the killing of one person by another – in legal parlance this is homicide. This thesis contends that these adaptive “solutions” might explain why people commit homicide in certain circumstances. In this sense, these explanations broadly align with “motives” for certain types of homicide. In some cases, such motives might constitute justifications or excuses; in others, aggravating features. The criminal justice system in New Zealand is underpinned by an assumption of rationality which is not always supported in individual cases. As a result, the legal mechanisms for apportioning blame in cases of culpable homicide are insufficient to recognise the different degrees of moral blame which can exist when one individual kills another. Therefore, the current regime for determining moral blame leads to inconsistent outcomes for factually similar cases, contrary to the rule of law which requires equality before the law. This thesis considers whether changing the definitions of murder and manslaughter will allow courts to legitimately recognise all relevant mitigating (and aggravating) circumstances in determining guilt. It will also consider whether there are other options for reform that might better deliver justice in the round. If law is to be relevant, it must reflect current knowledge about why people act in the ways that they do. If the law does not reflect science, it moves too far away from the realities of the community. Looking at homicide through a “brain sciences” lens can give us a better understanding of the psychological mechanisms involved in homicide, and allow for the formulation of an evidence-based approach which leads to a better appreciation of the degree of moral blame involved in particular killings. From this it follows that the criminal justice system will be better placed to appropriately respond to those degrees of moral blame. Three types of cases, in particular, illustrate that presently not all defendants charged with homicide are treated consistently: young defendants who kill; victims of violence who kill their abuser; and defendants who kill children. Defendants within these categories might demonstrate the same degree of moral blame, but the outcomes in case disposition differ wildly; or outcomes may be the same for very different degrees of moral blame. Inconsistency of outcomes means that a fundamental requirement of the rule of law is absent – the requirement of equality before the law. When elements of the rule of law are not upheld, justice is not delivered. This thesis argues that if our legal system recognises, in its application, different degrees of moral blameworthiness, then it would be as well to be upfront about them: the court’s “commiseration [should be] actually codified in the law’. The thesis therefore recommends a series of statutory amendments to the law of culpable homicide, including the creation of a “degrees of culpable homicide” regime with attendant defences, as well as a range of lesser offences.