"That can't be. Only Serbian Christians can be war criminals."
The existence of the ICTY and the trials of Croats and Bosnians proves differently, though I know the Serbians have complained about the laxity of sentencing of Bosnians and Croats.
There have been cases were Croats have confessed to mass murder of mostly civilian Serbs, such as Miro Bajramovic who said he was personally responsible for killing scores with his own hands, which neither the ICTY nor the mainstream press dares to touch. The man he commanded him, Tomislav Mercep, is still a player in Croatian politics. There are other Croat murderers of Serbs, such as Branimir Glavas of Osijek, who ordered torture and murders of Serbs, and yet who are to this day in leadership positions in Croatia and unwanted by The Hague.