Did you ever hear about the Ristovic family - Serbs - who were murdered at their home (in a part of Sarajevo controlled by the Muslim forces) while they were eating lunch by members of the Territorial Defence (Muslims)?:
This crime took place at the very beginning of the war, on July 8 1992 in Gornji Velesici and, according to all assessments had wide ranging consequences, above all influencing accelerated departure of Serbs from that part of Sarajevo. The cruelty of the crime was especially surprising, because six persons were murdered in the middle of the day, in their own house, while having a lunch. The victims were: Radosava Ristovic (61) and her sons Pero (born in 1951) and Obren (born in 1955), their sister Bosa (born in 1961), Danilo Ristovic (14) and Mila Ristovic. Dusko Ristovic was wounded. Stojanka Mastilo, a friend of Mila Ristovic was also in the house at the time of the crime. She hid behind a couch and was not hurt.Two days later, on July 10, Oslobodjenje published a short and terse statement by the Ministry of Defense and the Chief Headquarters of the Armed Forces of Bosnia-Hercegovina which, with small mistakes, listed the names of the victims. According to this statement, the crime was committed by "three unidentified murderers who drove away in a Volkswagen 'Golf'; the car had no number plates and was marked with the insignia of the Territorial Defense".
http://mujweb.cz/www/kutija/ristovic.htm
Not sure if I'd call July the "very beginning" but I see your point.
The Muslim forces certainly did have their nasties as well. The BiH 7th division who fought the HVO was largely foreign and demonstrated all the love that we've come to expect from jihadists. That said, I refuse to group BiH Muslims in with that lot.
I think the ultimate difference between the crimes of the Muslims/Croats and that of the Serbs is that the Serbs had the heavy advantage in heavy weapons like T-55s, APCs and howitzers. It's a lot easier to feel understanding towards someone when they're outgunned and being beaten down and surrounded. Who knows - if the BiH had the heavy guns they may well have behaved much the same as the Serbs.