They weren’t murdered, they died under conditions that existed in POW camps on both sides of the war — proximity, lack of nutrition and disease. 1860s simply wasn’t a great time to end up in a prison of any kind if you wanted a long life.
The conditions in the federal death camps where adequate food, clothing and medicine was not a problem were entirely avoidable.
Read “to Die in Chicago” about the awful Camp Douglas or read about Point Lookout or Elmira aka “Hellmira” and tell me the perpetrators shouldn’t have been charged with murder. Men were rightfully convicted at Nuremburg on less evidence.