Well, I don't know who committed the most atrocities -- didn't keep score. I do know all sides in the conflict committed plenty. (Serbs, Croats, "Bosnian Muslims" and "Kosovar Alabanians.") I've never been in Chechnya, but from what I understand, that conflict has been most grisly with atrocities committed on both sides and both sides using snipers.
If you're looking for a flipped-out former merc, well, all above-mentioned ethnic groups from the former Yugoslavia have gone on to be mercs in African countries (sometimes associated with the FFL, sometimes not) where children as young as, sometimes younger than, the boy shot on Monday are often armed combatants. There were also some German mercs working for the Croats (don't know they were snipers) and a few Dutch mercs on several sides. Also the occasional Russian "volunteer" on the Serb side. And we all know about mercs or "jihadders" from various Moslem countries operating in Bosnia. Some are still there.
It's also possible the shooter didn't consider a 13-year-old as a "child." After all, he's well, er, not like the rest of us.