Who said anything about exterminating the Garleans, or being guilt-free and blameless? Genocide is the Garleans' thing - our goal is, at the moment, just to push them out of Gyr Abania.
There are no real survivors in war, just the guilty and the dead. The problem is that, even without the whole Shinryu business, Ilberd still got his war. He forced the Alliance to choose between either laying down their arms and dying for nothing, or holding fast and dying for something. The Empire isn't keen on negotiations, and even if it were the likelihood of them being open to talking things out would reasonably be close to zero since they believe the Alliance launched an unprovoked offense against their fortification. There can be no peace until Gyr Abania is reclaimed or Eorzea is under Imperial heel, and if we have to deal with the war Ilberd foisted on us we may as well do so on our own terms. Other options do exist, but with the Garlean soldier crapping his pants when Zenos did nothing I don't foresee it happening before the end of 4.0.
Yes, by choosing to continue the fight the blame rests on the Alliance's shoulders - but there is no other real option, save to sit around and wait to get steamrolled by the Empire.
We too bear some small responsibility for this. Had we but apprehended or killed Ilberd back in Halatali, none of this would have happened. (I'm still not sure what Yugiri brought Hozan and Doware along for if they didn't cover the exit.) What's done is done though, and all we can do is deal with the consequences of our actions (or lack thereof).
Also, while this is that sort of game, we don't yet know we're going to come out on top. I'd say it's reasonably certain, but not definite.
He sparked a(n admittedly long smoldering) war and then committed suicide instead of living with the consequences of his actions. That's about as cowardly as it gets.
And again, we were left with no real option, unless you'd prefer being put under Imperial heel without a fight.