Maybe the fighting, but I think the Mujahadeen probably killed more civilian Serbs in undefended towns and those Serbs within Muslim population centers. Also, the Bosnian Muslims had many Serbs in prisons/secret prisons, and Mujahadeen participated in the tortures and killings. Just as the Sandzak Muslims were allowed to visit these places - like the Silo at Tarcin - and beat the Serbs, the Mujahadeen did likewise.
Besides, there is photographic evidence of their crimes against Serbs on the net - the three headless Serbian POWs, for example.