what would be a theme statement for "when power corrupts" with textual evidence from night?

I think the rest of your line would be "power corrupts absolutely". The Nazis had complete power over the Jews in the camp. Absolute power reaches directly into the shadow part of the soul. Absolute power has no checks or balances so people tend to exploit their darkness. They inflict pain on others because they can and this evil desires feed upon themselves and drive people into even more depravity.