Your prose struck this perfect tone of heavy-hearted weariness throughout. I found the details of plot and action somewhat hard to follow at times, and I think you could do better descriptively than relying on, e.g., "this creature looked like an old-earth alligator". I found it sort of discordant that the narrator is supposed to be a decorated war veteran who has participated in other colonial invasions (or at least, that's the direction the story took me with regards to his background) and yet he seems to have never before even considered the idea that they're wiping out civilizations. Still, that doesn't detract from what is overall a good exploration of the theme and a powerful and evocative story.