If it can be proven he committed a crime while doing his 'duty', then he should be prosecuted.
I'm sure he deserves all the empathy, understanding and sensitivity he gave those under his 'care'. However, I have a personal method when it comes to decision making. It's called the mirror test; if I can look myself in the mirror in the morning and honestly say my decision was right, then it is the right decision.
Gitmo is a little different to Nazi death camps. The US soldiers there, as far as I can see, can walk out at any stage. They could take my test and decide either way on a course of action - stay or resign.
The Nazi camp guards had little choice - although some enjoyed it a little more than others. If it can be proven that they exercised barbarity within their own executive power (i.e. without specific orders), they should hang. It really is that simple.
Individual 'creativity' in a death camp among the guards, should be punished, if proven.