Let's a assume for the moment that a murder has been committed and we all agree that for justice to be served the perpetrator needs to go to jail for life without the possibility for parole. Suppose further that he is a young man and his father does not want him to have to spend the rest of his life in prison, the dad offers to serve the young man's prison sentence for him. If we allow this to happen has justice been done? Obviously not, you can't get off Scott free just because someone else is willing to serve your sentence for you. We wanted to put the son in jail for a reason, we want to remove him from society and we demand he be punished.
Well, this is an atheist blog, so I'm sure it's pretty obvious where I'm going with this, let's think about the Jesus story. According to the Christian narrative, we are all sinners and deserve to be tortured for all eternity. But in an act of love Jesus died on the cross and paid for our sins. As long as we are willing to accept this gift, we are free from hell and get to live in the eternal paradise of heaven instead. Apparently justice has been done. This makes absolutely no sense, and I would argue it is similar to the situation described above. We all deserve to go to hell, Christians say this all the time, just like our murderer deserves prison. Jesus gets us out of hell by dying for us, which Christians claim is justice served. However I'm sure they would not let the father serve his son's jail time.
Although on second thought this analogy isn't very accurate, the trade proposed by the father is much more equitable than the one involving Jesus. The father was going to trade one life sentence for another, I suppose it's not completely equal as his life sentence would be about 20 years shorter than his sons would be, but it's pretty close. Jesus on the other hand canceled out an infinite amount of torture by dying for 3 days. Since the ratio infinity to 3 is zero, we can't make it completely analogous, but let's do what we can and say the father instead offers to serve 3 days in prison to cancel his son's life sentence. It wasn't justice before, and we are going in the wrong direction. Furthermore, the deal with Jesus doesn't just get us out of eternal torture, it gives us eternal paradise. We deserve punishment, but Jesus paid for our sins and we get a reward for it. So in the case of our murderer, he shouldn't just gain freedom, he should also be rewarded, say with a million dollars.
Let's see how this deal looks now, a 20 year old guy is convicted of murder and sentenced to life in prison. The father comes in and says he wants justice to be served and is willing to sit in a prison cell for three days in exchange for his son not having to go to prison and should instead get a million dollars from the court. Oh, and one more thing, if anyone else is convicted of murder from now until the end of time, they get the same deal provided they are willing to believe such a deal was struck.