Gavel being banged.

The two parts of this policy should be considered separately. On homicide law reform, the policy seems to be (a rather belated) acceptance of the Law Commission recommendations from its 2006 report Murder, Manslaughter and Infanticide; though first-degree murder is narrowed to intentional killing only. The policy here is well-tested and has received broad support from academic and policy audiences. Indeed, there is no reason (outside of election time) that this should separate the major parties. 

The recommendations on sentencing are more controversial, not least because of the current problems with prison overcrowding (i.e. all parties should be careful about increasing prison sentences without considering the future impacts and capacity implications). The focus on domestic homicide cases follows a review from Clare Wade KC, in which concerns are raised about the sentencing of domestic abusers who kill their victims. Action on sentencing may well be justified in light of this review, though care must be taken to remain targeted: similar increased sentences may not be justified in all domestic homicides, e.g. in cases where a victim of abuse kills their abuser.