Think of them more like the Spanish Conquistadors. They are a military organization expanding their empire and we are the small native species they will have to wipe out eventually.
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The Fallen and Cabal both seem to have motivations separate from the Darkness. The Cabal just seem interested in expanding their empire as Earth's seemingly defenseless Solar System looks ripe for the picking. The Fallen Houses seem to be the remnants of a once noble, unified civilization that fell from glory. They appear to pillage the ruins of other civilizations just out of survival. I sorta feel bad for the Fallen...
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What about the Hive or Vex?
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The Hive and Vex are a bit more straightforward. They both see the Darkness as some sort of god/all-powerful being. The Hive are just servants of the Darkness. However, the Vex are still pretty enigmatic in their longterm goals.
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I think the fallen are closer to pirates. They don't just come to pick up the scraps, they'll attack you and take it from you too.
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Your assessment is pretty spot on.