Here's an analogy. Not a good one, but bear with me.
Mein Kampf was made when Nazism was still irrelevant, in the 1920s. It was banned in some countries in Europe AFTER World War 2, but we shouldn't erase it just because we hate what the Nazis had done. We keep it existing because A., we understand the context behind the book, so we'll try to prevent history from repeating by actually learning what happened, B. Nazism is banned in multiple countries nowadays anyway and C., it gave us insight into Hitler's way of thinking, despite him being a horrendous tyrant.
I hope you'll understand what I mean.