Small theory about what Dark Sun might be doing (I'm not entirely sold on this one but I wondered about it)
So Dark Sun overpowered his Moon, ripped out his intelligence, and put it into himself; he hates all Moons.
From the way he interacts with Sun, he seems to want Sun to turn against his Moon. He calls him naive, says he needs to "grow a spine," etc.
Dark Sun says "all Suns hate their brother, they just don't know it yet."
At the very least, he wants to make Sun hit a breaking point where he gives up and becomes as cold as him, and turn his back on all Moons/everyone around him.
But, maybe New Moon/Nexus has a bigger role to play in that than just being a catalyst for Sun to snap -- something more similar to the fate of Dark Sun's Moon?
Is it possible Dark Sun is building New Moon/Nexus up, NOT ONLY turning him evil to make Sun hate him, but to give Sun a suitably intelligent Moon to rip apart and eat the intelligence of when his breaking point comes?
It's been a thing lately where Old Moon is consistently putting down New Moon's intelligence. But New Moon was holding himself back because he was trying to be a better person so that his work didn't hurt anyone. Dark Sun has given New Moon the space to embrace his most evil Mad Scientist thoughts where he's probably going to be frighteningly competent and Old Moon will have to eat his words. Not only that -- Dark Sun is showing New Moon all the horrible things he's learned how about during his travels, such as his witherstorm, all his Dark Matter nonsense, and who knows what else. This means New Moon will not only reach his greatest potential for himself, he's also learning all kinds of wicked, forbidden things from Dark Sun.
...almost like Dark Sun is fattening New Moon up with all that knowledge so that Sun can take it for himself when he finally snaps and eats New Moon's brain.
(It would be like a gift. Even though Sun seems to struggle with self-hatred, he cares for other Suns, like in Nothing Happened dimension where he wanted that Moon to be nicer to his Sun. If that's a personality constant for Suns, then Dark Sun says he hates himself, but he would still care for other Suns even if he's lying to everyone and himself that he doesn't and want to "help" them not be in such bad situations. His way of "helping" our Sun would be to break him away from his attachment to all Moons, and then hand-deliver him a hyper-smart Moon to eat the intelligence of on a silver platter.)