Time heals all wounds, DHW. It seems to have healed the wound of the victim. So I don't see any problem with tempering justice with mercy.
Indeed, the victim seems to have displayed plenty of maturity in all this. If she had taken the advice of posters here, she would have spent her life in agony, in bitterness, in rage, burning for revenge on the perpetrator. Her life really would have been ruined in that case, the perpetrator would have been able to ruin her life.
Now I don’t know anything about her, except that she wants to forgive and forget. But it seems to me that she has already forgiven him long ago and moved on with her life. And indeed, that is the proper attitude when a great injustice is committed. Rather than letting it fester in your mind for decades, to let it corrode your mind like acid, to live, to exist only for retribution, for retaliation (which may never come), it is much better to forgive the criminal, put him out of your mind and get on with your life.
So even if your daughter says to forgive and forget, you would go after the perpetrator and kill him. How is that going to help your daughter and her kids (your grandkids) if you are put away for life in prison for murder?
That may satisfy your personal desire for revenge, but the consequences will be very tragic, for your daughter and for your grandchildren.
Your daughter has moved on, she doesn't want to revisit the sordid affair. If you are charged with his murder, the whole affair will be revisited in spades, people will be talking about it for months, It also won't do your grandkids any good if it is known their grandfather is a convicted murderer.
So what is it going to achieve, except it will satisfy your hunger for revenge? Isn't that an act of pure selfishness?