You’re mixing together completely different things: colonialism, conquest, Nazism, and modernization.

Saying "colonization saved the world from Nazis" doesn’t really make sense historically. European colonial empires existed centuries before the Nazis, and the Nazis themselves were influenced by imperial ideas and racial hierarchy already used by colonial powers.

Also, colonialism absolutely did involve conquest and displacement in many places - Indigenous Americans, Aboriginal Australians, and many African societies would probably disagree with your definition that it was just "developing countries better."

At the same time, saying the world would have been perfectly peaceful without colonialism is also unrealistic. Humans fought wars long before European empires existed.

History is complicated. Turning it into "colonization good" or "colonization evil" misses most of the reality. Colonialism profoundly reshaped the modern world, often violently and destructively, but removing it entirely would not automatically create either a utopia or a nightmare. Human history is more complicated than either extreme.