I'm inclined to agree with those who would attempt to distinguish between the physical action and the ultimate responsibility.
To me, dictators or military leaders may not personally kill anyone, or at least any large numbers of people; their underlings carry out their orders and do the actual killing.
So like pumpkin, unless there are individual exceptions I'm not aware of, I would say that Cromwell, Hitler, Mladic, Assad, etc. were guilty of genocide, but that the New Model Army, the SS, the Serbian fighters, and the Syrian armed forces committed genocide.
Or rather, in those specific cases perhaps 'war crimes,' though some like Hitler and Mladic were also guilty of genocide, which I think of as mass slaughter carried out away from an actual battlefield, or committed by people other than combat troops.
Assad's exact guilt with regard to genocide remains to be seen, when we learn what happened to the 80,000 out of 120,000 who disappeared into prisons and have not been heard from since. Fortunately, human-rights groups are keeping careful records, in case they never reappear.