Since when is murder forgivable?

When is someone allowed to kill without any punishment? Apparently following the civil war the south was granted "amnesty" which means forgiveness, so they were basically forgiven for the 360,000 deaths they inflicted on the north. I think that is absurd, do I think the south should have died for it, no, but just forgiving them was a very light punishment. It is in my opinion that the south should have faced sanctions like countries face today from the United Nations.