Ask any Serbian how they feel about NATO and they will tell you the horror stories of how innocent civilians were intentionally targeted by NATO when they were Yugoslavia. Russia pushes back against NATO, so naturally they side with Russia.

My Serbian friend has told me that During the WWII, in Croatia there was an organization called Ustashas, and they were killing Serbs, Jews, and even some Croats. After the war the Ustashas became a terrorist organization. Some of them planted a bomb in an airplane during the 70's when plane exploded in mid air and crashed in Czechoslovakia. This same terrorist group supported Ukraine in 2017, and even just a week ago these Ustashas went to help the Ukraine. So I think there are plenty of reasons why Serbs want Russia to prevail. They want revenge.