Correct me if I'm wrong, but I think there was no United Nations Organization by Hitler's time. The United Nations was a post-war organization, if I recall correctly.
The League of Nations was the peacekeeping body by Hitler's days, and they were considerably weak since they didn't have armies and they were not backed up by the US or the USSR. They were headed by the former world powers of France and Britain, who were not as powerful as Germany, the US or the USSR. They could only apply sanctions and they had some humiliating failures with the Manchurian and Abyssinian Crises.

As to the sanctions that would have been applied to the Nazis: I think the sanctions would have been harder had the entire race had been annihilated, as it would have taken longer and more atrocities would have been commited. The Nuremberg trials, I think, dealt with the Nazis in a fair enough way. Having said that, I don't think you can blame the Germans as a whole for the atrocities of few. They were desperate after the Treaty of Versailles and Hitler lead them out of that desperation, and that is why he was a god-like figure to them.

As to your other point, I have always believed that people should look after their own businesses, and if a country has problems too difficult to solve, then instead of asking for help to the UN or the US, I think the country should try even harder to solve the problem by itself. It is more practical, I think, but again, I am biased since I am an active anti-unitedstatesman. The United States or any other developed nation, for me, should not act at all when another country finds itself in trouble. It is simply not their place. However, if you disagree with me then please don't reply to this last statement because I don't want to deviate this thread and turn it into a debate.