Domestic Violence; Is there ever an excuse?

In my book "The Breaking Point" I touch on the subject of domestic violence in a scenario where the wife murders her husband. In this particular story, the husband was not abusive which is normally the case. After writing the fiction story "The Breaking Point", I did a little research on women in prison and found that an alarming amount of them are there due to crimes of passion, and more alarmingly the majority of the inmates are African-American. I found that to be very sad. Women as a people tend to be very emotional beings, we are also very strong in the way that we love hard and will go to any lengths to protect (or in some instances keep) the ones that we love. We will often stay in unhealthy relationships because we believe if we hang in there and love just a little harder then things will change or get better. Unfortunately, that better day rarely ever comes and then that's when bad things start to happen. At one of my booksignings, I met a young woman who told me about her friend that was incarcerated for murdering her husband. Intrigued, I asked her why her friend murdered her husband. The story was that the husband had been abusing her for many years and she would not leave for fear that he would kill her and her children. Then one day, the woman's pregnant sister was over visiting when he began asaulting her. The sister tried to break it up and the man then turned on her. In effort to protect her pregnant sister, the young woman stabbed her husband and killed him. She was sentenced to fifteen years in prison. Was that a justifiable sentence for the crime? Should one penalty fit every crime or should the circumstances evaluated case by case? What do you think?