I agree. IOTL the Yugoslavian position was quite ambiguous. I write recently some wikipedia pages related to Italian divisions of WWII, and there is an indications what several groups in Yugoslavia already supported Axis (Chetnics and Croatians being most prominent). Throw Bulgarians into the Yugoslavian mix, and Greater Yugoslavia will partition Czech together with Hungary and Germany.
The Chetniks were not "pro-Axis". They were formed as the national resistance to the Axis invasion, and did quite a bit of fighting. The Chetniks were largely ethnic Serbian, and it was the anti-Axis attitude of the Serbs which provoked the 1941 German invasion. The Germans offered a reward for the capture of the Chetnik leader Mihajlovic.
However, the Communists formed the Partisans (only
after the Axis invasion of the USSR). Some of the people the Partisans killed as alleged Axis collaborators were only political conservatives. The Partisans also showed no scruples about provoking Axis reprisals if those reprisals were directed against Chetnik supporters. This led to clashes between Partisans and Chetniks, and to some Chetniks cooperating with the Germans (though never Mihajlovic).
Another kernel in the nut was the animosity between the Croatian Ustashe and Italy, which seized Dalmatia, previously part of Croatia. The Ustashe were known to murder Italian officers traveling alone. The Ustashe also murdered many ethnic Serbs. Italy, for its part, provided arms to Chetniks for use against the Ustashe.
But the Partisans were opposed to Italy, and denounced this
de facto alliance.
The Croatian Ustashe were explicitly allied with the Axis, being awarded a puppet state of their own. But as noted, this state was truncated by Italy, and the Croats were rather sawed off about that. They continued the Axis alliance, but only because it was the only path to any sort of independence for them.