Eveville wants to invade, subjugate and enslave some small nation in another part of the world which we will call Utopia. However Eveville's evil intentions are found out by Utopians before it takes place. Utopians clearly have a right to protect themselves against such an attack and subsequent gross violation of their rights even if it resorts to deadly force. The freedom lost from slavery is worth dying and killing for I think most will agree. However the people of Utopia are peace-loving and lack the means and weaponry to adequately defend themselves in case of such an attack.
Through some black market dealings, the Utopians were able to attain a nuclear missile. But Unfortunately the only nuke they could get was a cheap one and thus not accurate and it has a .5 chance of exploding in Switzerland.
Let's also say that the Utopians have tried all possible means to 1. resolve the issue diplomatically, 2. tried all means at getting international support to solve the crisis and to shield them, 3. use of threats, 4. tried all means to obtain either less deadly but equally effective weapons or more accurate weapons. All these attempts have failed and they are left with the last resort: either launch the bomb, their only means of defense that will no doubt kill hundreds of thousands of Evevillians if hitting its target, or become slaves to them. Let's also say that if they launch the missile, the aggressive war waged by Eveville is guaranteed to end with a defeat for them. Let's also imagine that the Utopians have only a small window of opportunity to launch because Eveville has nuclear ability and is about to take out the only means of defense available to Utopians through a strike on their only nuclear stockpile but that Utopians have preemptive justification through credible threat from Eveville that they will invade and can now strike at Eveville.
Switzerland obvious does not want to be nuked and if Utopians launch, there's a 50% chance they will be the one's who get fucked. Switzerland is a neutral country and though against Eveville's intentions to enslave others, do not wish to try and force them to stop their practices of enslaving others because that will put them at deadly a conflict with their neighbors.
Since the Swiss are not involved in the threat to Utopians, and are (indirectly) threated by them despite the fact that Utopians do not mean to, it would seem that the Swiss have a right to defend themselves against being potentially attacked by a nuke. They may even be permitted to use deadly force against the Utopians to prevent them from defending themselves against the Evevillians by using the only means which happens to also seriously risks the lives of others.
So now we have a situation, and a tragic one indeed if there ever was one, in which two peaceful nations may, in some weird sense, be permitted to attack each other and commit massive atrocities against each other through no fault of either countries (rather it all stems from the fault of the Evevillians). Knowing that the Swiss may try and prevent them from defending themselves through force, the Utopians may now justly attack the Swiss in turn and vice versa.
Moreover, since the Swiss and the Evevillians are close allies, are the Evevillians now justified in attacking Utopia on the reason that they are protecting their allies from harm? Notice that the original sin in which this vortex of spiraling potential violence spiraled out of is not an actual violation of rights but simply an intention (to enslave and subjugate a people).
I have exaggerated the details obviously but I think many real world situations not only in international law of peoples but also more generalized in common everyday moral situations are analogous to this. That is, from an original sin, we have magnifier effects that affects everyone. As the old Jewish saying goes, when you save a man, you save the whole human race but this seems to be the converse, when you harm a man, you harm the whole human race (or at least many others not apparently associated with that man).
I think there are many real examples of instances where no apparent rights were initially violated or even things obviously wrong committed but that some actions responsible by some peoples created situations in which it was inevitable two groups of people would come to commit massive atrocities against each other and furthermore, it is not clear if any principle of justice will be capable of having a say in the matter.
Consider the ethnic conflicts in Darfur during the early part of the 21st century. Most scholars of the region consider that these ethnic conflicts occurred because the people in the region experienced the worst drought in their recorded history. Moreover, the reason for that drought is mostly agreed to be global warming caused by the carbon emissions mostly from wealthy nations. Due to this drought, many people had significantly reduced agricultural productivity and many groups that relied on raising livestock had to encroach on the land of other groups to maintain their livestock and feed their own people.
Many of the wealthiest nations that have contributed the most to global warming also were the ones that denied these groups assistance when they asked for it before it all came to violent conflict. They argued that they were not obligated to give and offer assistance (which they weren't at least under current international law). This example is slightly different than the one involving Utopians but there is an underlying thread. 1, that often violence spirals out of control and 2. that what makes them spiral in the first place, an original sin, may not actually be a outright obvious rights violation; it may merely be the credible threat of or even less obvious, the gluttonous use and abuse of a public commons (natural resources in the Darfur case) e.g. But the underlying theme is similar; that sometimes hopeless situations where massive atrocities are committed by groups against each other and there is no moral fact of the matter as to who is in the right and who is in the wrong among certain groups and how to begin to solve such a problem. It's just a sad situation for all.