In today's standards, That would not be ok. However, A person whose family was just killed will not consider today's standards in his next actions. It is perfectly fair for him to get revenge. The murderer did not consider his family when he murdered them, So why should the victim. If the victim does not mind stooping down to his murderer's level and suffering the consequences of such, Then it would be fair and equal for him to return the favor.
If a dog killed your pet's hole entire family tree I think I'd kill that exact dog (don't ask me why i'm using dogs because I don't have the answer) it's like karma, But I wouldn't kill that dog's hole family tree. If a raccoon killed my cousin i'd kill that exact raccoon but I would not kill every raccoon that passes my house, And I think the victim deserves to kill the person who ended their family
The murderer's family has most likely not been involved in this in any way. Killing them is no better than killing an innocent person. It's not their fault that they are related to a psychopath. Killing them just makes you as bad as the murderer. I would have no problem, However, With choking the hell out of whoever killed my family.
The killer could have just just gotten a women pregnant and left them. They could be living their lives separate from the killer. Some John Wick could go with a pencil and take their lives because the killer took his wife.
The killer would not even care if they got killed for his sake because he cares about nothing.
Then the person having the revenge would have committed a horrible crime and killed innocent people all for nothing.
Just get the killer responsible and leave others out of it.
I am assuming that the case is as simple as presented ( that is the murderer acted alone). The family did not commit the crime. The murderer is the criminal, And should face justice. However, The murderer's family did not commit the crime. Who is responsible under the law? Only the one committing the act is guilty. A person would not be justified in the revenge killing of the murderer's family.