I found this thread which has good answers, but I wish there was more.
Were the concentration camps a secret or did Hitler talked about them?
When did the world really start to know about the death camps to acknowledge their existence and talk about the horror of Nazism?
And more importantly, does that mean Nazism was not considered to be a dangerous ideology until people really knew about the Holocaust?
This post by u/commiespaceinvader covers this topic in detail, including the facts that the Allies knew about the Holocaust as early as 1941, the deciphering of Hofle Telegram, which detailed how many Jews had been killed up to the end of 1942, the work of European governments-in-exile, and the ways in which the New York Times and British newspapers reported the Holocaust.