Regarding the Holocaust, the Germans were assiduous in their record-keeping. The 'Final Solution' was monitored to ensure the 'new methods' of genocide were as effective as possible. In fact, if a death camp was not delivering enough death according to 'the figures', military inspectors arrived, the Kommandant was sent packing, and a new one installed, with fresh ideas of a better killing system. Records are available from these sources. It would seem also that, in spite of the enormous and horrifying death toll, there were survivors of the 300 plus extermination camps, and surely their testimonies count for something? The young man who killed at the age of 20 is still alive. His girlfriend is not. If the Courts deem one is of adult age and guilty, one must pay the price for one's deeds. Anything less would be an insult to the deceased's family. He is fortunate not to have paid the ultimate penalty for killing someone. I assume a death sentence didn't pertain in his State.
The 'common denominator' in all this punishment/time business is the retribution on behalf of the victims and their families. There's far too much concern for the perpetrators and not nearly enough for the sufferers. An old man who killed is still a killer. An ageing entertainer who once abused children is still a paedophile. Old crimes are still crimes. And punishment should IMO still be meted out.