there is a large confusion in the law and lots of grey areas. If someone was attachked then I would use reasonable force to protect them. Its the definition of reasonable that is the problem. If someone was mugged and I shot the perpetrator then that would be murder. If on the other hand someone was being raped at knifepoint and I sneaked up behind them with a baseball bat and smacked them with it that would probably be justified. There would be no charges probably even if I killed them. If however i then repeatedly struck the attacker it would be back to an assault or attempted murder charge for me. People in the UK have used weapons to protect themselves and it been ok under the law on many occasions it just has to be an appropriate level of action. The problem is that this is subjective and open to the opinion of whichever judge hears your case. (not saying I agree with this just trying to say what the law is)

In the UK there have been several cases of people suffereing domestic violence snapping and stabbing their partner multiple times and beeing freed as what they did was seen as "reasonable" in the circumstances. If someone was being raped and snatched up a kitchen knife and killed there attacker then it would be investigated to see whether the woman should be charged but she almost certainly would not be (has happened)