Who was the only person held accountable for the mistreatment of soldiers at prison camps?

Andersonville prison commander Capt. Henry Wirz

 The prison commander was executed. However it is probable that he had little control over most of the things that he was accused of.
"Although he is often said to be the man who was in charge of Andersonville and everything that happened there, in reality he had very little control. Management of the camp was splintered."

I think  it's 1(Gettysburg proved the South was not going to win. and 2(They did not want the U.S. to harbor bad feelings.

I could be wrong but that's my opinion