You're wrong. The Nazis labelled the Serbs (like other Slavs) as subhuman. They helped establish a fascist
Croatian state, where they carried out systematic cleansing of Serbs from Yugoslavia (Crotians were not targeted by Nazis since they claimed a fictitious theory of originating from Germanic Goths or Iranic Scythians). Most of the socialist Partisans that fought against the Nazis were composed of Serbs (but of course they also had Croats/Slovenes/Bosniaks etc). Bosniaks (the Muslims) were labelled as Croatian by that fascist regime, and the Nazis (especially Hitler and Himmler who viewed Islam as a Martial religion established several Waffen-SS brigades that had majority Muslims and was blessed by Berlin-resident Palestinian Grand Mufti, Haj Amin Al-Husseini). The largest and most famous of those was the
Bosnian one.
Of course, during the breakup of Yugoslavia, there had been a lot of massacres by hosts of sides, be it Croatian or Serbian. The Serbian genocide of Bosniaks at Srebrenica is obviously the biggest of these events, but it wasn't motivated by religion, but by ethnicity. Modern Serbs do not have cordial relations with Bosniaks or Albanians, who are primarily Muslims, but they don't have good relations currently with Catholic Croats, or even Orthodox Bulgarians. You have to remember that the Balkans has been a hot mess for many decades, most of these ethnic groups were neighbours, and even intermarried during the peak of Yugoslavia.