Whether or not his actions are the only way to end the deathly regiment (or all the other crap the confederation does) is up for debate. But Abraham clearly believes that it's the only way, he's said it in so many words and, given what has transpired so far, he has a point. It wasn't Thorn's choice to stop trying the peaceful path, the confederation forced it on him by declaring him an enemy of the state. Geming Chu would be the case in point giving the lengths the confederation went to to prevent his election. Does this prove it is impossible to stop legislatively? Not with certainty, but Abraham isn't the first Thorn who has tried to stop the Deathly Regiment and I suspect he doesn't want to be the next one to die doing so. When the other guy is willing to kill for it you can either kill him back, surrender, or die pointlessly. It is only with the benefit of hindsight that people would be able to judge whether sealing the Lands Below is "Necessary", all Abraham can see is that it is the best, and probably least casualty intensive, way he has to hurt the confederation. You might as well argue that the Italian Campaign or the sinking of the Tirpitz were war crimes because it can't be proved that they were absolutely "necessary" for the defeat of Germany, such certainty is not possible in the midst of war.