It would make sense if the assassins were sent by monsters/the demons.
Killing every royal able to use the star power that is a threat to them, starting from the weakest.
So humanity will lose the possibility to return in the future.
Zion was useless, but he could sire a heir to the royal power, I'm ready to take bad my words on the bad plot, IF, the assassination was a order of another monster in the royal palace or someone that got tricked by them.
Now for why the MC thinks is weird a power so similar to his exist in the future, wouldn't be normal to expect your heir to inherit it? If not it means is not a power that he can leave behind, that means it was analyzed and replicated, maybe they used his corpse.