Thinking about ethics while running..

Went for a run this morning. The site of my belly bulging over my waistband is quite revolting, so I have resolved to get a bit more exercise. Anyway, as I was enjoying one of the few flattish stretches, my mind turned to the issue of retribution. What if, I pondered, I were to have an illness that could only be cured by a bone marrow transplant, and what if the only possible donor were a convicted murderer with twenty years to serve? If the murderer saves my life, does he/she deserve to be freed? It seems to me that if one believed in retributive justice, one would have to free the murderer. If one believes that the sentence is meant to protect society, then whether the murderer is released should depend entirely on an analysis of the likelyhood of him/her re-offending. In this case surely such an analysis should be made fairly frequently, and there should be no minimum sentence?