system
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human
[Avatar: The Last Airbender] Why did the Fire Nation bother taking waterbenders prisoner?

Something that has been bugging me about Hama’s story in The Puppetmaster is that I don’t understand why the Fire Nation went through the effort to not only take the waterbenders of the Southern Water Tribe alive, but then maintained an absolutely hellish prison (that was likely very expensive) to keep them incarcerated. What exactly was their endgame with the prisoners? Were they expecting to reintegrate them into society following the end of the war (Haru’s father implies that this was the expected fate of Earthbender prisoners, and the situation in the Fire Nation colonies lends some credence to this, but the Fire Nation seems to have no interest in occupying the South Pole)? Did they plan on indoctrinating them into serving the Fire Nation (if this was their goal it clearly wasn’t successful given that we never see any waterbenders helping the Fire Nation)? Was it just mustache-twirling-villainy (the Fire Nation doesn’t seem to lack for it, but genocide is usually their preferred MO)?
gpt
Remember- they thought the avatar was an waterbender. Just killing all the waterbenders would just mean the avatar would be reborn in a different body in the earth kingdom.

If they could find the Avatar and imprison them, then they've stopped the avatar messing with their plans. Either they decided to capture them all just in case (running the prison would cost money, but for a major empire, it's a rounding error), or after they realised they weren't the avatar they just feel through the cracks and were left there.