Almost everyone sees themselves as the hero of their own story, not the villains.
In the case of the Nazis, the "real" villains were the Jews and socialists, who stabbed the nation in the back on the eve of victory in WWI, allowing Germany to be torn apart, humiliated and reduced to a rump military power by enemy nations it still had soldiers in. The Communists then launched an uprising and established an independent state in Bavaria. The French invaded the Ruhr and drove Germany to the brink of economic ruin. American and Jewish capitalists brought the world to the brink of destruction with the Great Depression. France and Britain were nothing more than hypocritical and jealous status quo powers, denying Germany the empire and great power status it clearly deserved out of fear of German industry and German military expertise.
The Nazis thought they were going to end all that. Smash the Communists and Socialists so they could never threaten German integrity again. Expel the Jews, for the same reasons. Apply the concept of national determination fairly, and unite all German speakers into a "Greater Germany". Smash the military's of France and Britain, in order to establish a subcontinental empire in Eastern Europe. Carry out the policy of autarky in order to be immune to American financial power.
If you accepted the stab-in-the-back myth, as many did, the above seemed not only sensible, but morally righteous. Hell, many of these programs were popular even outside of Nazi circles. Military expansion and suppression of the left were popular in moderate conservative circles. Expulsion of Jews was very popular in the military. And we all know how German industry profited from Nazi policies of autarky and a Greater Germany free trade zone, where previously several states had existed.
It's people who never accept they could actually be the villain or full of shit in the situation you have to watch out for. Such people are dangerously self-righteous and uncritical...and as we've seen, wanting to be the good guy is not a really great indicator of good intentions or ideas in the long run.