I’m not a legal expert but I guess it would depend on the circumstances. In the UK if you are attacked in your own home (or indeed anywhere else for that matter) then you are perfectly entitled to righteously defend yourself, and if your life is in danger (or you feel that it is) then you are perfectly entitled to use deadly force. But if you ended up killing someone by doing that, then that would involve a coroners inquest. And if the coroner felt that your use of deadly force was inappropriate, (a verdict of unlawful killing) then you would be in trouble. This is what happened to that farmer who was imprisoned for shooting at a couple of intruders a few years back (and killing one of them). He got into trouble not for defending himself, but because he shot them in the back as they were running away from his property, and therefore they were not threatening him at the time.
In my state and probably most others that would buy you a world of legal crap.
The saddest story I read was the elderly twin sister who was murdered. He called her over to the cab and then shot her at close range. Horrendous and abhorrent. God bless her and all those who were murdered, their families and all the people of Cumbria.