I’ll do my best to explain my understanding of it! I don’t usually get lots of theory questions so this is some pressure, haha. The first part, his comparison of colonialism to Nazism is pretty strongly worded. Cesaire equates the methods that the Nazis used in enslaving and mass-killing the groups they considered undesirable with the same tactics he argues that the colonizers used against indigenous peoples. Therefore, to Cesaire, the true horror of the spectacle of the Holocaust, and the reason European society reacted so viscerally to it all, was because the tactics that Europeans had previously reserved for subjugating non-Europeans were suddenly unleashed on other Europeans, and that scared the shit out of them.
I think one really important thing to take from the first section is that Cesaire is reminding us that genocide was NOT a new concept to European society; they had been practicing it in varying degrees for three-hundred plus years. The Holocaust wasn’t the first event of its kind in the world, but it was the first time this had been done to other Europeans. THAT is the terrifying part. The violence isn’t problematic to European society; it was (is/will be) already dependent on performing it on other groups. The object of the violence, other Europeans, was the terrifying thing.
As for the demonization of humanism, I think he’s referring mostly to the moral hypocrisy of the thing. Humanism was often used to defen colonialism: Europeans felt they needed to enter these “savage lands” to save them from the backwards, immoral, terrible lives they were living. The humanist imperative absolved the Europeans of the guilt of subjugating others. The narrative that the colonizers adopted was that they were just trying to help. Which is, not coincidentally, the same narrative adopted by American interventionism.
I’m not sure if this is all true? I’m not an expert on this kind of thing, but I really enjoy discussing it, so I hope it helps!