I confess that, when I saw this thread, the Holocaust was what came to my mind. There had been genocides before (e.g. the Kaiser's attempt to annihilate an entire tribe in Sud-West Afrika (now Namibia)) and the Armenian population of Ottoman Turkey, but in both cases this was because they were perceived as enemies, on the one hand resisting German colonisation efforts, on the other as a potential fifth column. The Holocaust was a campaign of planned, industrialised extermination of a people just because who they were, not because of any danger they represented. It is, in my opinion, the worst crime in human history and it represents an absolute low point in human morality.