"The Darkling didn't do it for Grisha, he did it for himself!"
First of all, that doesn't make any sense if you remember how ridiculously little he gained for himself for centuries—he doesn't have any influence outside the little palace, any titles, lands, or even love and respect from the Ravkan population. You would think that the master manipulator aspiring for grandeur would have organized a cult of personality around himself and turned the whole country into a place of worship for himself, but no. He still serves the king, still fights in wars, still provides for his grisha, and is still considered an unnatural monster. I don't know about you, but if I were an immortal, super-powerful megalomaniac with otherworldly manipulation skills, I'd have everyone eating out of my hand and thanking me for allowing them to live in my majestic country. And if I came across a sun summoner, I wouldn't tell anyone. I'd simply lock her up, use her powers, and take credit for them.
But even if we forgot about it for a second and accepted that he did want power, glory, sainthood, or whatever—so fucking what? The fact is that he is the only person who bothered to build a safe haven for Grisha and had a strategy to liberate them. Not a single morally pure, selfless wanker apparently has lifted a finger to do anything similar.
A member of an oppressed minority running away from armed bigots or slavers will not give a single flying fuck about whether the guy offering him a shelter does it for himself. "I would rather starve, freeze, and risk getting lynched than accept help from someone whose motives are not 100% pure and selfless!!" said no one ever. The material outcome of your actions is way more important than your motives.
A person who donates to charity for clout still helps more than a person who doesn't donate at all. Moral purity doesn't pay the bills. Historical political figures, military leaders, and revolutionaries who brought about change also weren't saints and didn't have purely selfless motives, and people, driven to desperation by oppression, injustice, and indignity, didn't give a fuck.
And it's the same staggering hypocrisy again: Nikolai gets to be unapologetically ambitious, Alina gets to leave everyone behind because she "just wanted a normal life", Zoya gets to pursue power and fuck her fellow grisha over because "she just wanted to feel safe uwu" Their selfishness is proof of their humanity, but if the Darkling wants something for himself - he is a greedy monster who lost sight of his cause.