Fordola (spoilers)

[Reynhart]

At one point, there are ala mhigans who were born under the garlean occupation that try to ambush us - in their eyes, we are the invader.

No, they don't attack us because we're "invaders", they attack us because they're convinced we will lose against the empire, and that they'll suffer even more as an aftermath, so they try to stop us before.

There is also a side-quest where the locals are urging you to stop the alliance from executing one of the enemy commanders, whom according to them always treated them fairly and found a way to give them their lives back, going out of his way to ensure none of the grunts abused them.

Oh yeah, sure. Not all Garleans are douchebags. But the most important ones (Basically, the ones with actual voices) are jerks.

Furthermore, on an earlier side-quest there is also mention of how Gaius stopped eorzea from being attacked with chemical weapons, act he condemned - the fact zenos didn't pursue the same avenue goes to show that (at least to some extent) they too follow some form of war ethic (but that's just speculation).

I don't think so. Zenos just likes to fight. Killing masses with chemicals just isn't interesting to him. This is not ethic, this is just a whim.

Frankly, compared to Gaius and Regula, Zenos is just a really bland villain.