The Ustaše were so savage, even the SS was taken aback.
That says something.
You’re reading half the story (as is often portrayed during discussions of Europe at the time). Many countries faced communist revolutions and saw their monarchies fall, and saw fascism as a preferable alternative to communism; especially after the massacre of Catholic religious in Spain during their civil war (8,000 priests - including a dozen bishops - executed, along with hundreds of nuns, without the “western allies” lifting a finger). Hitler & Mussolini intervened to suppress the communists is Spain and elsewhere (with the help of former monarchists), while the Soviet Union supported the communist revolutionaries. Within a short time other populations faced the same choice; second-guessing their decisions seems easy today, but the fact is that the west wasn’t intervening to contain communism as they would during the Cold War. The populations of eastern Europe had no idea that they’d been bartered away at conferences between Stalin, Roosevelt, and Churchill, and they paid a steep price for that; the enslavement of those populations subsequent to WWII certainly vindicates their concerns about communists.
THIS IS NOT A DEFENSE OF THE PERPETRATORS OF ATROCITIES OR NAZISM; IT IS SIMPLY PUTTING THE WHOLE SITUATION OUT THERE.
The issues in the former Yugoslavia were complicated by religion, ethnicity/language, and politics; it was bound to be brutal.
Oddly enough, the west that awarded all of those territories to Serbia after WWI (creating Yugoslavia, with Serbia’s king as the king of the new country) bombed Serbia into submission to release them 75 years later - after treating those minorities that fought against Serbian domination as enemies in WWII.