One doesn't have to actively be committing genocide, placing their fellow human beings into giant ovens as a Final Solution to be like the Nazis. At least I hope that it doesn't have to come to that to make comparisons. Just representing the same kind of hate and bigotry the Nazis were about and seeking to create policies intended to oppress the people that you hate are enough to draw the parallels. If we have to wait until we're in the middle of another actual Holocaust to raise alarm bells, then what is even the point of teaching history? We point out similarities to the Nazis, the Soviet Union, the KKK, and other various monsters of history so that we hopefully never have to repeat this history.