"You have no right to judge me! " It's not a good line of defense in the court. "Might is right" and they are not the mighty ones.
It may make his status of 'soldier' discutabale and, therefore, a court can consider them as 'gangers' - illegal migrant, who illegally took the weapon and started the criminal activity. Yes, it highly depends on their motives. If the court decides that their main motive was 'money' - they may be considered as 'mercenaries', if 'hate' - they became 'hate criminals' or 'gangers', if their main motive was 'love' - they may be considered as 'volunteers' (and it is not a crime to be a volunteer at all).
Yes, Kievan regime allow foreign citizens to join their army or paramilitary organizations (in violation of its own laws). And DPR authorities consider it as another crime of Kievan regime.
You see, Americans fought against the Brits to achieve freedom. Ukrainians are fighting against the Russians to achieve their freedom. People of Donbass are fighting against Kievan regime to achieve their freedom, too.