Death-worshipping, pagan-heathen Europeans did. Followers of the prophet Hitler.
And death and destruction worshipping, blasphemous followers of Derr Fuerher, Mohammed.
And God, having given us the FReedom of choice that sets [Some of] us apart from the animals, loves us far to much to interfere with the excercise of our will!
Shalom! Shalom!
Why? Check how many Jews have won Pulizers, Nobel Prizes, how many diseases Jews have cured - how many of the great American Broadway classics were written by Jews, how many pop favorites were written by Jews...and according to the wingnuts Jews run Hollywood and the media, and on and on. Jews had a Hell of a run in the 20th century. And will continue to do so.
I believe God's gift to man is free will. What man does with that free will separates him from the things that crawl on the belly.
If you are referring to the Holocaust, those who committed it are the things that crawl on the belly. It is they God curses. And their modern-day enablers as well.