BBC Merlin is a really excellent reflection of how how important framing the narrative is; how winners always come across as the good guys in history.
Because, objectively speaking and removing myself away from having watched the show and knowing the characters, if I was some random dude in Camelot/if this were happening now I would support Morgana & co. over Arthur & co.
In universe, the perspective is Morgana is standing up for magical people's rights and advocating a change to the system. Yes, she's going about it violently, but in universe that's the norm and, at the end of the day, its still essentially a form of activism (and the most effective one given context, again).
Meanwhile, the view of Arthur is that he inherited the throne and generally speaking he changed nothing. Yes, he perhaps reduced the active violence against magical beings and yes he brought some regular ppl into the nobility via. the Knights and marrying Gwen. But he hasn't actually changed anything for his people. Their conditions are still the same; the conditions of magical beings are still that they're illegal/persecuted.
Whilst as a viewer you get the idea that Arthur is better than Uther (he saves Mordred as a kid, went out of his way to help Merlin a few times, went against Uther when it came to the Knights/Gwen), as a citizen he's not really done anything to make your life better. The status quo still stands fully intact; there's just maybe a bit less murder going on on the daily. To offer a historical comparison, best I can suggest is Elizabeth I compared to Bloody Mary - it's in the title, one was just a Queen whilst the other was a murderous one. Arthur was just a king whilst Uther was a murderous one.
And, again, as a viewer you know about Albion and the prophecy. That inclines you to him because that's a good end. But he never achieves that, he dies before any of that happens. There is nothing to show for that in universe.
And so, back to framing a narrative. Arthur & co. are the protagonists, heroes and winners of the show. At the end of the day, they defeat the constant magical threats against them (which really undermines the idea that magic isn't evil). The Kingdom is more peaceful and prosperous under their Reign - they are portrayed as having defeatung the evil of Morgana. Its akin to an old monarch being portrayed as having defeated a peasant's revolt or, in more modern terms, Britain defeating activists/freedom fighters in colonised nations. And, particularly with the latter, the way history framed that as a good victory (even though it's objectively not).
Point is, BBC Merlin takes a character who based on many modern perceptions (e.g. In my case I'm anti-monarchy and pro-equality yet still as I watch the show I root for Arthur & co. as it draws me in) wouldn't really be the good guy and pegs him a hero. Just like how historical victors are always the heroes of the story.