I think that Simon didn't want to save the king, he just wants him to turn on the “selfish commoners” to realize all he was doing was for nothing, to regret wanting to help his people and sacrificing himself for that.
He ultimately hates the king and his optimistic mentality and wants to teach him a lesson, because he thinks he is dumb. He feels superior. And he just wants to prove his point.

I think the artist must have changed the plot halfway through. The rebellion just doesn’t make sense. It feels like they backed out of making Simon actually evil. I think that it could have been explained though. Like if Simon had started the rebellion just to make the royal family turn on the “selfish commoners” but it got out of hand and they killed the royal family. So Simon used some forbidden magic to turn back the clock? But the story doesn’t use magic and instead uses some weird mind control syrup. Idk. It just needed to be more coherent.
But also seriously? What is Nils eating? Bro did not age in over 10 fucking years. He looks younger than Noa and Alex’s kid!