Someone kills your family what are you going to do about it? Kill the killer before he can do it to anyone else? Or let it slide because you want to be the "better person?"
Make effort to contain the killer by whatever degree logically possible. If they killed my family in front of me, with murder clearly in their intent, then the path is to kill them. If they killed my family and fell down the stairs and broke their back, I'll do my part by calling emergency services. I don't 'need' to be a murderer in almost all cases that will ever happen to me, while I can understand that lower standards mean something.
Levitical (etc) law, for example, probably mostly did have a point for most people in most ways in its day. And that's it. Its standards on 'life and death' have to be understood as imperfect and not a way to improve truer justice in the world.
I'm not killing someone solely because they killed anyone I love. I dislike the idea that anyone would become a murderer through the death of myself, at least. I don't want to inspire those ideas and actions in others.
Of course, I could be tested in fire at any moment. I could end up a hypocrite, or an example of my ideals. I dunno for sure until it happens to me.
Please don't kill my family.