The group behind the assassination was crna ruka, black hand.
Former Yugoslavia was many states ethnicities and religions in one.
Slovenians and Croats as Catholics. Croats as ustashe (they were allied with nazis), bosniaks were ethnic Muslims and 60% of the population was Muslim but not practicing or knoweldgable, mostly traditionalist/cultural (they would say my great grandmother went for hajj but wouldn't know anything about the religion), and the serbs were orthodox christians -- the cetniks with some serious ethnic pride mixed with religion but very vile people, who I don't think ever read the bible.
Kosovo or Kosova as the Kosovars call it was where Serbs fought Ottomans and one of their 'legendary' rulers was killed, lazar, so they have alot of legends and stories that revolve around this even though it was like a thousand years ago, and they would have all kinds of writings in literature, theaterical plays about Muslims being demons and 'turks'. Their perception of being Muslim was being turk.
The conflict that began in the 80s was fueled by these 'legends', pseudo-religiosity and ethnic pride and ultra radical nationalism.
The US and EU just played their role. Former Yugoslavia was a power player, independent block nation that had a developing independent industry, so they played their card how to divide the region and give false promises to one side and the other. Not to mention their fear of being allied with Russia, even though Tito fell apart with Russia in some respects.
There was a lot of tension from communism then later adapted socialism, suppression of religon, a mixed bag.
The cetniks/orthodox christian serbs were the ones behind the massacres and genocide of Bosniak Muslims and Croat catholics. The Croat catholics initially tried the same as the cetniks but didn't succeed. All for a 'great serbia'
There is also tension from ww2 where the croats were allied with hitler while the bosniak/serb side was with the 'partisans' which were fighting nazis.
I have some montenegran blood myself and I can speak the slavic languages