I don't think the author is implying that the Aztecs were better or the same or worse than Tlaxcala or Cortez conquistador. The neutral tone is adopted for European and Native American ancestors alike because of the separation of time not cultural sensitivity. And so we adopt this historical voice because we realise that the past was bloody and awful and the same society that had Cicero also had Crassus crucifying 6000 slaves along the Via Appia during the same lifetime. And the same people who built towers of skulls were enslaved and wiped out by Cortes.
But honestly it's not a competition. You adopt the neutral tone to avoid endless whataboutery.