Watching an older New Tricks...
I try not to apply American standards of self-defense to the rest of the world, because we are kind of insane, but I am not convinced that even in the UK 'I came home and found my friend tied up and a guy with a knife torturing him, I punched the guy once and he died' would actually result in significant legal trouble. Generally the 'eggshell skull' rule only applies when the underlying violence is illegal in itself.