Would you say that to someone who had just tragically made that mistake? It's not like it's an uncommon occurrence.
You're weaving in your argument. Either it's responsible gun ownership or its not.
A responsible gun owner will not blindly blast away without assessing the situation as I previously posted. OR.....the irresponsible gun owner will go Gunfight at the Ok Coral and in which case should never have owned the gun in he first place.
You're arguing an ACTION rather than the method. It could be equally argued that "He jumped out of bed at the sound in his house, saw a shape in the dark, and grabbed his (gun, knife, baseball bat, mace, nun-chucks, ray gun, etc ) and began to ( shoot, stab, swing,spray,Kung fu,stun or disintergrate,etc ) and tragically found out it was his loved one.
Now, what would I say to someone who shot a family member as you asked? To their face I'll say " sorry for your loss". In the privacy of discussion with other people it would be " if that's what he was going to do then he should have not had a gun in the first place".