Although the Soviets certainly were mass murderers before WW2 and British and USA still sided with them.
Hitler was elected under the pretense that he represented socialism that was friendlier to the middle class and workers. This way people could get their socialist improvements to the economy (which was shit due to bad world-wide economy and war reparations) while still having defense against the the sort of upheaval and murderous destruction that the Reds represented.
The take-home lesson of WW2 shouldn't be that 'The other side was evil and we won'. Because the entire Eastern half of Europe and most of Asia was submitted to governments that were incredibly evil due to the Soviet victory.
WW2's lessons are meaningless without WW1. They really are effectively the same war. The treaty of Versailles and the humiliation of the German civilian government are directly responsible for the rise of power of the Fascism in Germany.
The take-home lesson of the 20th century wars is that massive murder and atrocities are only possible because people obey their governments. That 'The people' cannot discern true evil running the state until it's far too late.
Because evil doesn't show up saying "Elect me because I want to gas the Jews". They gain power by promising what you want. By telling you what you want to hear. And once they gain power then it is the average person's willingness to obey authority and carry out orders is what turns shoe makers, engineers, and doctors into mass murderers.
Which is the sort of thing that is happening in many parts of China.
Always remember that in Vichy France when they rounded up the Jews for the holocaust it wasn't the Germans troops that went around arresting them. It was the French police that rounded up people to be put on those trains. It was under the order's of the French politicians. This problem of obeying governments is not something that is limited by national borders.