From what I understand, the Ustaše (I think they were Croatian), were so brutal, they sickened the Gestapo.
Tito held Yugoslavia together, but that unity couldn't survive his passing. They've been fighting each other for so long, that I suspect the original reasons are lost in antiquity.
Tribalism is very human, and results in the worst fights.
I grew up in Africa, and saw what tribal hatred looks like. Not pretty, but Africa doesn't have the monopoly on it.
We have tribes all over Europe, and America. The behavior is exactly the same, everywhere.
I'm always open to be corrected, but as far as I know nations that formed Yugoslavia haven't actually fought each other meaningfully before WWI and even in that war Slovenians, Croats and Bosniaks were not involved independently, but as subjects of Austro Hungarian empire. There certainly wasn't the kind of animosity displayed as for centuries between France and England or France and Germany (as just two examples). It was really WW2 where one can observe the viciousness beyond fighting one's enemy.
I have my own views why things went south so badly and I agree with you that it was inevitable for YU to fall apart, but I find the often expressed argument that people living in our parts always did this very unpersuasive.
In the West, we have a term "Balkanization." I had assumed that it's fairly old.
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In Africa, many of the really vicious tribal wars, date from the colonial times.
The colonial powers leveraged old tribal animosity. It was a way to keep their colonies from concentrating on them.
Demagogues, conquerors, and dictators have always known how to leverage old resentments, and fan them into a conflaguration.
Like I said, this behavior is very human, and we're seeing it on this side of the pond.
The Balkans are mountainous. (In fact, "Balkans" was the name of a mountain range before it was the name of a region.)
France, the Netherlands, Germany, Poland, Ukraine and Russia (among other countries) in contrast lie on a large plain called the European plain, which makes invasions easier, so the governments there had to pay attention to military matters more. As an exception to this general rule, between 1945 and 1989 the world's 2 superpowers took a keen interest in Europe and one expression of this interest was to promise to come to the aid of their "side" of Europe is any country on their "side" was attacked, to which the countries of Europe responded by focusing on things other than military matters, which is a sign that the Europeans are not naturally warlike, but rather responding to incentives imposed by geography.