As an aside, the Warrior of Light has been shown to have the equivalent of PTSD. Garlean combatants have also been shown to have the equivalent of PTSD.

- Livia begins to lose it after realising that once again, everybody she cares for is being cut down right before her eyes.
- There's a soldier in La Noscea who hides away in a hut. I forgot his name, though.

Insofar as magic and environmental destruction is concerned, airships have been shot down by Eorzean forces. I doubt the land scorched by the twisted wreckage is going to sustain life for quite some time. One can argue that it is a necessity in war, though.

Which brings us back to Black Rose. When it was used, it was used at Elidibus' behest - not Garlemald. The exception to that is when it was used in the future, though that was done after much hesitation and as a last resort when Eorzea's forces where literally about to take Garlemald's capital. A scorched earth policy, at that point, makes sense. The Garleans would have lost most if not all of their territory beyond it. The people within would fear for their lives (and wish vengeance for their loved ones who had been cut down).

So, yes, its use is perfectly understandable in such a context. The unintended consequences were unfortunate, though given the Eorzean's inability to find a peaceful resolution to the conflict one can argue that they should not be surprised that such a thing would happen. You don't slaughter your way through large swathes of territory, murdering any and all that stand in your way and then complain when a last desperate act is made.

Again, the Eorzean's are the aggressors in such a scenario. Which, strangely, does not seem to be acknowledged by many around these parts...