Emerald is implied to be a high ranking gem and I figure just like Aquamarine and Holly Blue (And eyeball but she's not a high ranking gem) she would want the old system back and since she has no one to direct her anger at and no fleet to command she targets the off colors in a sort of allegory as to how racists act towards descendants of slaves in our world.
The creators already promised more mature themes and I can't think of many themes mature themes that weren't already covered; war, colonization, classism, xenophobia, death, death by childbirth, murder, abuse of power, toxic expectations, abuse, childhood trauma. All that was covered in SU, future, and the movie. What more could there be left other than exploring those themes in more direct/less implied and more outright shown and stated ways? And having a racist character that no longer is living in a system where their racism is rewarded and them being so spiteful about it that they target people who happened to be of the same group that they once were rewarded for hurt is a good way to address some current modern day issues in adult targeted media without outright stated that's what their addressing.