Mankind has been taxed with the job of punishing crime. God, however, punishes sin. For crimes of this nature it is NEVER society's duty to forgive. They have not been told to do so.
We must however, pray that these evil ones will not be punished both for the crime AND the sin. God's punishment will be more terrible. Let them repent. Let us tell them to - that is the duty of believers.
Nobody can forgive sins on behalf of others!