Genocide Only by the Army?

As the BBC has reported, Joseph Mpambara - who is a Rwandan Hutu - has just been found guilty of torture by a Dutch court. He was not found guilty of charges of war crimes, however: despite his role in ordering the torture and brutal execution of at least two women and four children, Mpambara was not found guilty of war crimes "because he was not part of the Rwandan government army fighting Tutsis." The implications of this conclusion include concern about the rise of a fundamentally different kind of warfare; the 21st century world is one in which more people are coming to commit horrific acts against civilians even if/when they are unaffiliated with a governmental army. Does this lack of affiliation legally imply that civilians cannot - for whatever reasons - commit acts with the "intent to destroy, in whole or in part, a national, ethnical, racial, or religious group" and thus not be found guilty of genocide or even war crimes?