Self Defense - Our morals come from the rights claims we agree upon when we form the social contract. According to Hobbes, the only right which we cannot sacrifice completely is the right to self defense. If one’s life is threatened, he/she has the absolute right to defend, and the aggressor effectively sacrifices their right to life by threatening that of another. Repeated domestic violence can be said to actually threaten the victim’s life. It traps them in such a cycle of physical and psychological damage, that it effectively robs them of their life. Therefore, their deliberate use of deadly force to escape the situation is morally permissible. To make it more explicit, suppose the person has been captured by a serial killer. The serial killer continually rapes and beats the victim for several years before killing them. Would we not admit that it is morally permissible for the victim to use deadly force to escape the situation? Why is repeated domestic violence any different?
Vigilante Justice - It is morally permissible for individuals in society to react accordingly when government fails. In the case of repeated domestic violence, it is impossible for government to come up with an adequate response because this violence occurs in the private domain. Justice systems the world over have failed to develop appropriate recourse systems which efficiently and adequately respond to domestic violence. It is impossible to record the goings on in every home to protect against this crime. Therefore, it falls upon the victim to react. This is the same reason we retain the right to bear arms.