I'd need to mention that "I don't consider you [a person]" has historically been used to further -ist and -phobic schools of thought, propaganda, and outright violence. The more correct term would be "othering", and it's generally meant to, well, paint other people as something "other" than human (and by necessity "less than" human), and so any crimes against them wouldn't really be a crime. In this case it's simply that Emet considers us "less than" Ancients and thus not worthy of living.

Now, yes, the sundered are "less aetherically dense than" Ancients, but the whole theme of the story is that they're still people. They have feelings and lives and hopes and wishes, and deserve to live said lives to the fullest so their lives aren't worth less than the Ancients.