Well, in my sad experience, it takes a seriously huge blow to kill someone -- even the head can take a huge knock without lasting physical damage. Smashing them once or twice over the head with a frying pan will probably knock them out cold, no more. And, if not, then there was no intent to kill and you'd be seriously unlucky to be prosecuted.
My guess is that, for all that, you would lose sleep over it. I know I would. It fundamentally would never matter that they were the ones who started off by breaking the law -- as soon as you have any reason, rational or otherwise, to believe that you could have done something different, oh that will eat at you so much. And killing someone, taking a life, that will be the biggest cause of doubt of all. If you didn't lose sleep over it I'd be amazed. And perhaps even jealous.
The basic point is that the law as it stood covered this pretty well already. Once someone breaks into your house you can get away with a great deal, and rightly so. The cases where people have been prosecuted, the defendants went too far.