There's an issue that Riot has admitted to a ton, that Demacia/Noxus is supposed to be this "morally grey" thing, but Demacia's flaws have super rarely, if ever, been canonically acknowledged, and "Noxians did a horrible thing to me" is the backstory of like 20 champs.
Demacia's bad thing is supposed to be that they're classic Medieval society, with all the shitty side effects of that. Totalitarian rule, racism, sexism, etc. But we...never really see that. Jarvan has never been depicted doing anything really bad. The "totalitarian" laws totally accepted Sivir, a former Noxian mercenary, into their city (This is pre-retcon I'm citing), and basically just gave her a slap on the wrist when she got drunk and went on a joyride around the city. (JoJ) They're openly accepting of other races, with Poppy being a major ambassador (Again, old lore for this part) and they have several high ranking male and female champions.
Really the only negative aspect of Demacia we saw in the old lore was in Lux's Judgement, where they've sorta got this whole shady brainwashing thing going on, which is pretty terrible! But it just...never gets mentioned again.
Noxus' good side was meant to be that they were founded on this idea of strength allowing you to rise above if you worked hard enough, but Quinn is a random farm girl in Demacia who managed to do exactly that. Meanwhile Noxus also has the gladitorial slave pits, the chemical weapons, the random invasion of Ionia because "lol why not", the slaughter of Freljordian barbarians for kicks, the genocide of the Minotaurs, sensationalized public executions (
) and THEY LIVE ON A GIANT SKULL MOUNTAIN.
With the new lore, it seems like they're trying to give Demacia a more blatant dark side.
Fiora's lore is pretty much all about sexism running rampant in the city-state, and turns her dad from a cheating asshole into a tragic sacrifice who got screwed over by House Crownguard. I don't think the sexism bit lands well, though, considering that there are a load of Demacian lady champions who seem to command a lot of respect, so it's a bit of a contradiction.
I think Taric's update is a little better about it, mentioning that his love for nature, art, and other precious non-Demacian things is a "potentially treasonous philosophy", which I think ties way better into what was implied with Lux's Judgement: Demacia is brainwashy as all hell. I think that's a good angle to push on if you want to make Demacia a bit more shady.
To actually get to the point of the whole topic, because of that, I'd be interested in seeing a villainous Demacian champion who was one of the people in charge of the military indoctrination. Like, go for the whole shady inquisitor meets drill instructor vibe. These people basically have the job of molding children into killers for the crown, I think that would be a really interesting direction to go. Of course, yeah, corrupt official could work well too, but I think the issue with a Ragou type character is that they don't really fight, and you kinda need that to be a champ. You could always add in some magic skills or something like that, though, maybe go for a WoW Archbishop Benedictus "corrupted clergyman" kind of deal.
Tl;Dr: There aren't really any outright villainous Demacians, because the whole Demacia/Noxus conflict is sorta still in the process of being redone, but there are a LOT of interesting angles for one.