Should forgiveness by the victim lesses the penalty for the guilty?

  A few years back a family was returning home from an evening out together when two gunmen confronted them, open fire with guns, and killed the mother, one son, and seriously injured the father and shot the second son in the arm.  While recuperating in the hospital the father was filled with hatred for the people who killed his family members and he thought of ways he would like to exact revenge.  Then he began to think of various scriptures and realized that he should quieten his angry and forgive the perpetrators, whomever they may be.  It was only after his change of heart did he discover that the shooters were two of his injured sons friends and they were acting on that son's instructions.
   One shooter received a life sentence, the other shooter received 15 years and the son who planned the whole affair received the death penalty.  The father wants the sentence reduced to life imprisonment.  I understand.  No one wants to see their child die, no matter what they have done.    But that decision should not be his nor is the sentence part of forgiveness.
  Forgiveness is not for the perpetrator but for the victim.  When we fail to forgive and hold hate in our hearts it's like a poison that slowly robs us of our happiness.  Forgiveness is never about letting the perpetrator get away with his crime.  The stat's role in the sentencing is that of protector of the populace.  The guilty parties sentence should not be increased because the victim hates him nor lessened because the victim has chosen to forgive him.
    I am not against the death penalty but I do believe it should be used only in the cases where it has been demonstrated that the guilty party is capable and likely to continue a life of crime and there is grave danger to the public that the guilty party may kill again.  Serial killers, spree killers, and mass killers all fit that category.  But often they get lessor sentences in order to secure a conviction or to gain information that is vital.
    The father in this case should just step away lest he run the danger of turning his hatred for the killer into forgiveness and then into hatred for the state that executed the killer.  Forgiveness has come to naught.