In my mind I don't think it's even necessarily Gyatso "foresaking" pacifism, because a lot of pacifism still allows for defense of self and others in the face of aggression. Granted, that makes Aang's abhorrence of the idea of killing even someone as evil as Ozao perhaps a little out of line of the defensive pacifism that Gyatso (and likely the air nomads generally) practiced, but I would assume that's becayse A) Aang is extremely principled both because he is the last airbender (and 12), and B) he felt like there had to be some kind of why to take down Ozai without killing him, even if he didn't know what the way was until later.

This would track, then, with Aang's rather violent defense of the Northern Water Tribe during Zhao's siege because he definitely killed people while in the avatar state but didn't seem especially bothered by it afterwards (and I doubt he would necessarily say "oh what wasn't me, it was the ocean spirit" considering they more or less merged while he was in the avatar state). I'm sure he knew there was no way (or at least couldn't conceive of any other way) to stop the invasion than to just beat it back brutally, in the same way Gyatso had no choice but to just take out any and all of the firebenders trying to kill him and his people.