My favourite bit of BG3 lore is that Withers is legitimately responsible for the Dead Three, but he's probably too embarrassed to tell you, so every time you ask him to elaborate he just gives you a very stern, "Noooo."
I also love that the reason he's responsible for their uprising is because he got bored. He literally got bored of his position as Lord of the Dead and wanted to retire, so when these three morally questionable humans came looking for godhood he was like, "Hmmm. Yes, okay. Here. Take my portfolios. Fight over them. I don't care. I quit."
So after bowling with skulls in a friendly competition to decide who would get what portfolio, they took up his powers and wreaked havoc on the world. Only at that moment did Jergal, AKA Withers, AKA our precious Bone Daddy think, "I'm just now, internally, asking myself, in quite a worried way, whether I might've made an error."
So he joins your merry band and watches your escapades, calmly twiddling his fingers while you clean up his mess. He's happy to lend his aid, even to the point that he'll bring Durge back to life if they reject Bhaal, even though he technically shouldn't. But he's Withers. The rules don't apply to him. If Ao doesn't like it, he can descend from the Heavens and say it to his rotting face.
And the reason he saves Durge isn't necessarily because he likes them or because he's a morally good entity (though one certainly could make that argument), but because he wants to add insult to injury. He steals Bhaal's child with a big smile on his face, dubs them his Chosen, and praises them for rejecting all the power they were promised. But of course, he still doesn't tell them who he is—or rather who he was.
Then, when all is said and done, he throws Tav and their companions a cute little party. No one knows it's probably half a thank you party and half a "Withers is bored again" party. And if anyone misbehaves, he'll get irritated and whisk them away. Because how dare they? He put a lot of work into that.
And at the end of it all, he walks up to a mural of the Dead Three and basically goes, "Lmao. Thou didst fuck around, and thou didst find out." Just savagely roasting them.
And then poof!
He waves them into non-existence.
