The Nazis clearly knew they had been very naughty or they wouldn't have taken efforts at the end of the war to cover up and hide things and run away. What were they hoping to achieve by allowing photographers and cameramen to record actual mass executions and the like?
It sounds like your question is dealing with two time periods: the efforts to cover up their war crimes in 1944-1945, but many photos of war crimes come from before that period, a period when Germans thought they would win the war. They were a part of the trophy culture of Nazi Germany.
Wendy Lower's Hitler's Furies discusses the safari-like atmosphere that prevailed, particularly in the East.