This is a huge and subjective question, and there's no way that I can begin to say everything that I want to in this post, so here's a few thoughts:
- 'the Germans': not all German people thought the same way about every issue we categorise as 'evil' during this period. It would be a better question to ask how actions we class as very evil suddenly occur;
- anti-semitism was present all over Europe before the war, including in England. We tend to forget because it suits our national narrative as 'the heroes who saved the Jews'.
- The Nazis looked for intrinsic characteristics, and classed people like communists etc as 'intrinsically evil' - in classing all Germans, then or now, in the same way we are doing what they did. Evil is an action, a way of being; people are brought to believing that it is good/right/necessary.
- The Second World War was not an isolated series of events that suddenly happened between 39-45. Remember that Adolf Hitler had been in power since 33, and a political presence since 29 or so; remember that the First World War and the Boer War had contributed to settling up the political climate which allowed his rise; remember that the financial crash in the twenties deeply hurt Germany's economic powers... there are so many contributing powers, and many of the thing that shock us, that we would class as 'evil', were already being built up or existed in other countries before the war. Concentration Camps are a British Boer War invention.
People have offered some really good points on this, so I won't go into any more specifics - it's just worth question the premises on which you base your question.