In death penalty cases should the victim's family be able to choose whether the murderer lives or dies?

No, I don't think so. I think an impartial jury is best to decide. When we are most blinded by fury and vengeance we do things that we later regret. Some may regret that they sentenced an individual to die. One reason may be that they feel the death was "too easy" and that the person should suffer in prison. Others may at first think that life in prison is worse than death, only to find later that they didn't consider everything and wish they'd sentenced the person straight to death. As for the law being a tool of retribution, it IS a tool of retribution. Look at the fines and penalties people have to pay, when their actions clearly didn't cost anyone that much money - even the courts who tried their case.