There are a lot of reasons why the Nazis are histories bad guys; for a start, they were mainly responisble for ww2 which resulted in the deaths of millions of people, from all over the globe. During that war national feeling was of course very high, so as well as people feeling more patriotic towards their own nations (as happens during any war) they also focused hatred on the enemy. Even though we are a few generations on many are still personally affected in some way.
Added to this the Nazis came at a time when communications were improving, literacy was at an all time high and most people read newspapers, many people even had televisions or saw newsreels in the cinemas and saw the horrors of the war, and after the war ended they saw pictures of the concentration camps and mass graves. Perhaps our sense of national grief (and possibly guilt) was somehow sated by the idea that we did the right thing, that the Nazis were, to a man, inhuman and needed to be killed for our own safety and that of others.
But when you look at it subjectively, disgusting, evil, brutal as the Nazis were, it is hard to view Britain and the USA and USSR are the good guys to any great degree. Britain was responsible for a fair amount of slavery and genocide, the USA had almost wiped out the entire population of the land they took over, and were at the time heavily influenced by the KKK who in many ways were similar to the Nazis (and supported them too) and the USSR was holding its own pogroms against the jews, although their reasons were religious rather than ethnic cleansing.. But in short, history is written by the winners, so Hitler, Mussolini and Hirohito are the triumvirate of evil, and Roosevelt, Churchill and Stalin the Holy Trinity. Well, almost..