The answer seems so obvious that you will most likely believe I'm "splitting hairs" here.
If I witnessed a physical attack on any of my children, who are all adults now, I would do anything in my power to stop the attack, including killing the attacker, if that was the result. If a daughter told me her husband/boyfriend/whoever was physically assaulting her, I would demand she put this scumbag out of her life, pronto. I would not blow his head off his body.
What good would I be to her or anyone if I'm residing on death row, for instance? As long as I'm capable to provide it, she will always have a safe home, with me.
OK. Obviously if an attacker could be subdued without killing him, I would do that. The question was at the endpoint of a series of questions that my relative had answered 'no' to. It was an extreme example, but I would kill someone else, if necessary, to protect my loved ones.