I don't understand why this "must" be the way things are done; it seems way more of a stretch to argue this than to say that if the family of the murderer is innocent, choosing a punishment specifically based on wanting to make them suffer seems pretty messed up. I don't think claiming that innocent people who happen to be related to criminals should be forced to suffer is a universal premise; if you're going to claim it, you're going to need to back it up with an argument about why that's somehow more reasonable than "we shouldn't go out of our way specifically to punish innocent people".
> If we are going to shy away from punishing wrong-doers on the basis that the punishment won’t stop other people from committing the same offence then we might as well not send anyone to jail because sending people to jail has never stopped other people from committing the same offences.
If your goal is a 1:1 justice system where the guilty party suffers the exact same punishment that their victim suffered through their crime, doesn't that also imply that imprisonment should basically only ever be used as a punishment for kidnapping/holding people hostage? Do you punish a drug dealer by forcing them to buy drugs from the victim, or a fraud doctor to get care only from people without medical degrees? It's virtually impossible to try to define punishments like this in general, so I don't find it compelling that it's somehow the obvious way to punish murder.