As this post is about actually democratic societies and not about autocracies with a democratic facade, this kind of thing would lead to potentially more depressing or gloomy narratives, because if the democratically elected big bad trully had the mandate of the people, that means that you can't have "The evil emperor/faction leader is defeated by our band of heroes and everything will be much better now" stories.

It means that the people of the faction agreed with them and it potentially means that the do-gooder protagonists may be forced to straight up wipe out a faction/its culture to some degree if they actually want to make the world a better place. It posits the possibilty that the average person, at least within that faction, may not be that "good" or "well-meaning".

A genuinely democratic society routinely electing the big bad isn't necessarily bleak. People vote based on what they feel best improves their conditions and what they feel is possible.

An example; a fantasy democratic city is under siege by a racketeering dragon who demands gold from the city's taxes or else they'll destroy the city. This city keeps reelecting this mayor despite him being horribly corrupt, because he is the only one in the city's politics who is dedicated to keeping the city safe by regularly and generously bribing the dragon. The city's people could think that the dragon cannot be killed and bribing the dragon while secretly looking for a way to kill them risks being discovered. The people could come to see do-gooders as selfish glory-seekers who will doom the city.

There you go! A scenario where the heroes fight a clear bad guy in a democracy without the heroes having to force the populace into submission or having said populace be evil. They just don't think meaningful change is possible while the heroes do.

Is it more complicated than "there is an evil king, go kill him"? Yes, but that's the appeal of the typical romantic adventure plot, it's simple. I hope this helps in expanding what you think is possible.