Why should the victim of a crime be allowed to decide their accusers punishment?

HELP! I have a debate coming up where I have to advocate that the victim or the victims family should be allowed to decide the punishment of a criminal that has been found guilty. Every argument I think of, I can think of a counter argument. Justice is supposed to be blind and letting the victims emotions decide the punishment seems wrong but that's what I have to argue. I think I'm going to mention that the crime at hand, would have to involve murder then the family could decide. How do I argue that without it infringing on the constitution or it being eye for an eye? How do I prove eye for an eye can be a good thing? Oh I'm in so much trouble. Please help!