Not Nazis, worse. The Utaše. Comrade @WhyEssEff linked this the other week to me.

the barbaric croatian fascists, who were so brutal in their methods even the nazis were disgusted by them, for reasons much more empathetic than the fact that they were slavic.

German officers in Croatia and Bosnia repeatedly expressed abhorrence at Ustaše mass killings of Serbs, using words like “slaughter”, “atrocities”, “butchery” and “terror”, while citing hundreds of thousands of victims. Thus Major Walter Kleinenberger, officer with the 714th division, complained that Ustaše brutality “was in defiance of all laws of civilization. The Ustaše murder without exception men, women and children”. German Captain Konopatzki called the Ustaše Black Legion slaughter of Serb civilians in Eastern Bosnia “a new wave of butchery of innocents”. Ustaše “wholesale butchery” (Abschlachtung) of Serbs in Srem, in ISC-occupied Serbia, triggered German concerns of Serb uprisings. Lieutenant Colonel von Wedel wrote that in western Bosnia Ustaše killed women and children “like cattle” in a series of “bestial executions”. This is how Hitler's Plenipotentiary to Croatia, General von Horstanau, described the aftermath of slaughter committed by Jasenovac concentration camp guards in a nearby village:

At Crkveni Bok, an unfortunate place, over which about five hundred 15- to 20-year-old thugs descended under the leadership of an Ustasha lieutenant colonel, people were killed everywhere, women were raped and then tortured to death, children were killed. I saw in the Sava River the corpse of a young woman with her eyes dug out and a stake driven into her sexual parts. This woman was at most twenty years old when she fell into the hands of these monsters. All around, pigs devoured unburied human beings. "Fortunate” residents were shipped in terrifying freight cars; many of these involuntary "passengers" cut their veins during transport to the camp [Jasenovac]"

The German military even took the extraordinary step of trying the Ustaše chaplain, Miroslav Filipović, for the massacre of 2,300 civilians in 3 villages around Banja Luka in February 1942, including 52 children at a school. On March 3, 1943, General von Horstanau, wrote "Thus far 250,000 Serbs have been killed". General Lothar Rendulić wrote how in August 1942 he remarked to an Ustaše official that he could not conceive how 500,000 Serbs had been killed, to which the Ustaše replied "Half-a-million is a slanderous accusation, the number is not higher than 200,000". Other German sources put the total Serb victim numbers in the ISC, as high as 600,000 to 700,000.

the rationalwiki, radlib as it is, has a good article on them. https://rationalwiki.org/wiki/Usta%C5%A1e