Just wanted to point out that "working" with the sundered is pretty much explicitly stated by Emet-Selch in the last quest of 5.0 "Shadowbringers" as a response to you saying you will stop him
Emet-Selch:
"Did you now? One last do-or-die attempt to foil my plans before your mind dissolves into madness?
How very, very...heroic of you."
"In every single age, there is always someone who wants to stand up to the evil Ascians."
"Always the same arrogance, the same insistence that the world belongs to them. As if theirs were the only rightful claim, theirs the only existence worthy of preservation!"
"Even now, after everything, you refuse to see reason.
You think it unfair that you are subject to suffering? That your lives will be sacrificed for the ancients?"
Emet is tweaking at this point you can really tell he's slowly ripping apart at the seams the line delivery is really good but the important part is this-
Emet-Selch:
"Look at me! I have lived a thousand thousand of your lives!"
"I have broken bread with you, fought with you, grown ill, grown old! Sired children and yes, welcomed death's sweet embrace."
"For eons have I measured your worth and found you wanting! Too weak and feeble-minded to serve as stewards of any star!"
"Have your recent spats with Vauthry and his sin eaters taught you nothing?"
"Have you not learned that your ignorance and frailty beget only endless misery?"
"How long do you mean to perpetuate this farce? How much more must I endure your bumbling interference?"
The ascians are guilty of genocide from a sundered point of view and they aren't goody two shoes I assure you! But, Emet did genuinely try. As we learned from Tiamat, mankind without any ascian interference is going to fight amongst itself, the people of Meracydia fought over territory and only compromised once they went to a higher power in Bahamut who served as mediator. I can't fault Emet for believing that a world of inhabitants who kill each other at the drop of a hat is a far cry from the Utopia he once knew and loved.