It's good that you aim for consistency, but if your views do not contribute to a reduction in the use of force (disproportionate or otherwise), then how can they be justified in the first place? It doesn't matter how objective the criteria you employ for picking sides are: the fact of choosing sides contributes to increased violence, regardless of which side you pick, or why.
Nor should you. The rub is that the side you should be taking is that of peace, not that of either party to the conflict. What if "remaining neutral" is the most (or even only) effective way to cut down on the numbers getting slaughtered? I understand the visceral appeal of the violence in the conflict, but emotional reactions to such matters are typically counterproductive to the cause of peace (hence their popularity as propaganda amongst those invested in continued conflict).
Is the priority to punish one side for its misdeeds, or to prevent future suffering? Because those two goals are very often at odds with one another.
You've probably observed children getting into fights, with each side insisting that the other is to blame. And you've probably also seen how little patience adults have for this line of thinking, as they understand quite clearly that indulging those sentiments is a recipe for continued conflict, while the priority must be peace and forgiveness. The absolute worst thing that said adult could do is to endorse the justness of either side's use of force. Even if the adult is convinced that one side really did "start it," the correct course of action is for the adult to mete out an appropriate penalty directly, not to encourage one child to take violent revenge on the "aggressor." Even then, in my experience, both sides end up getting punished for having resorted to force, regardless of who, if anyone, "started it," and said punishment is typically (and rightly) more severe than that corresponding to the "causus belli."