Sometimes deadly force is what it takes to stop physical abuse. Sometimes there is no other choice. I can see circumstances where a child or spouse is in an abusive relationship and ends up using deadly force against the abuser. Sometimes the victim can't take anymore abuse and this becomes the only option at that time.
There are plenty of articles and cases of victims of abuse getting away from their abuser, filing a restraining order, only to be hurt or even murdered by their abuser later. If a victim is being physically abused or attacked and may have to use deadly force to save themselves, they should do so.
In my opinion, a person should be able to use deadly force in response to physical abuse. For example, if a man is hurting his girlfriend or wife and she cannot defend herself, she should be allowed to use deadly force against him. In addition to this, if a person is being harrassed, he or she should be able to follow the "Stand Your Ground" law and protect himself.
Deadly force is okay in response to physical abuse. This is because a person who is being abused is a victim of the perpetrator. Victims often endure abuse for quite a while before getting to the point they would use deadly force. They are often physically weaker than the abusers and to get rid of the abuse, they feel forced to sometimes get rid of the abuser him/herself.
What should be allowed in self defense is reasonable force in response to an assault. If a man hits his wife, the wife should be able to respond with physical force equal to that which she received WITHOUT the intention to kill. However, what should not happen is if a man hits his wife that she is justified in using deadly force, i.e. with a firearm, in response to this. That is manslaughter at the very least. An eye for an eye, not a life for mere physical assault or abuse.