FGO chapter america is, essentially, a conflict between the Celts (lead by Medb and Cu Chulainn Alter) against America (lead by Blavintsky and Thomas Edison.) At the fringes of this conflict, the player teams up with the third parties (Most notably Nightingale and Rama) to try to reach a proper solution to this mess. Yes, this is a real plotline in fate. I love this game.
Anyways as I was rereading it it struck me that we've got a lot of king characters. We've got Cu Alter, the Mad King. We've got Rama, the king of Kosala. We've got Edison, the Presi-king. Arguably we got Robin (the May king) and Nero as emperor as well, but I freely admit I didn't play fate Extra so I'm gonna push these two aside for the sake of this post. I'll come back to it when I actually know their deal. Unless I forget lol.
So. What does it mean to be king? The question isn't actually that easy. Cu Alter seeks to destroy America not because he wants to or likes to, but because this is what he believes a king does. Edison seeks to anihilate the current human order to change it for a better one, but it's also not his own desire; it's one born from him as a king, while he forces his people to go through unspeakable labor law violations for this ideal. Rama is the one who has to say the least about being a king, but he still says that him banishing his wife had nothing to do with his personal faith in her, and all about appeasing his people.
A king is someone who is beholden to their duty. Cu Alter puts it best: "Animals are free until they die. Unfortunately, I am a king." A king cannot do what they want. They can only do what they must. And sometimes what they must do sucks for absolutely fucking everyone, including them and their very people. It's a theme already brought up by Saber in Fate/Stay Night; Saber isn't happy being king, a good chunk of the round table isn't happy with her sacrificing herself constantly, her very country still fell under her rule so the good she did bring with her reign was debatable, but it was nonetheless her duty to carry things through until the end. Straying from the Nasuverse for a bit, but this is also not dissimilar to Antigone by Jean Anouilh, which depicts Creon as a guy who really, really doesn't want to kill Antigone, who is doing everything in his power to bend the rules for her, but ultimately it is his duty. He cannot derogate to it, and Antigone will not let him do so. He accepted to be a king, and that is his cross to bear.