Well, first of all, the second part of your imagining (i.e. "that that person happens to be REALLY perpetrating") is what I was trying to get at in the war example: until it happens YOU DON"T KNOW -- you simply don't know what is "real" in the intention of the other until the other shows you. Or until you find out through secret intelligence, I suppose. REAL intelligence...
But suppose you are right, that you guessed correctly, and that he was planning something. I think what we KNOW is that violence creates violence -- unless you indulge in the fantastic idea (fantastic as in of an unreal and twisted view of reality) that destroying every trace of the other (i.e. all of their friends and family and history) will succeed in stopping them. Otherwise, all that happens is a slowing down of their own plans for "perpetrating bad things". Or what appears to be a slowing down, anyway. I think of it as more like putting a blocker in the exhaust pipe of something. All of the exhaust is simply gathering invisibly inside the thing, and will soon explode with greater force and more chaotic results than a normal leaking of exhaust would.
So, IF genocide were possible --I mean, really possible in its intent: that by erasing from existence every single trace of the person/people you don't like, or that you fear-- then genocide is the only logical way of following through with the Un-Golden Rule, as I see it. And that is probably why you see so many leanings toward and actual attempts at genocide in the world. Unfortunately, genocide is not possible, it cannot succeed, because we cannot track down all the bonds that our "enemy" enjoys with others -- others who will seek some kind of justice or revenge.
This is why it is the Golden Rule that people who think about things long and hard end up affirming in forms all over the world. The Un-Golden Rule (and I apologize for being so blunt if you are someone who is in favor of it) is for pseudo-realists who are caught up in the fears and paranoias of violence, but because they live in a time or place when people need to rationalize their behavior, they struggle to come up with some logic that supports their actions. Or maybe it is just a part of them that is stuck in violence, and another part is wanting to get out by thinking and reasoning their way through it, but the violent part is so much in control that it is directing the reasoning part... I don't know.
But I disagree completely with the Un-Golden Rule.
Thanks again for the great question.