I think with their shapeshifting abilities the more Humane ones became more Human. I mean look at Rose Quartz. She loved humanity, an I would say wen't 99% human. I mean look at the skin tone.

And I bet something more insidious. The more rebellious gems TURNED their Former Allies into monsters. As dangerous as the monsters where, they where incomparable to the powers of the Crystal Gems. In a desperate war, they did the unthinkable.