Merlin is a show known for Became Their Own Antithesis, so much so that it probably has the longest entry for this trope. So, working on Word of God and whatever the show gives us, let's look at a few of their characters and evolution
MorganaMorgana's first episode had her at first be sympathetic for a poor woman losing her son, then get angry at Uther forcing her to celebrate, and then for the rest of the episode focus on how much she hates (alright, despises, but it will develop) Arthur, who had nothing to do with it, then turn up at the feast smirking. Wow. Way to plot out your character's entire storyline in the first episode.
To start things off, the first season. Morgana starts out extremely nice. She doesn't discriminate against magic users and non magic users, and she helps whoever we can. She saves a Druid boy. She goes with Merlin to save Merlin's village. She stands up to Uther for his crimes.
But then "To Kill a King" rolls around. The entry in the trope page there pretty much tells us all we need to know: Morgana starts out feeling sorry for Gwen, but afterward she turns the situation into an attack against her father and her. Do we ever see her comfort Gwen in this episode, or feel guilt that her own actions partially caused Gwen's father's death? No, and that's extremely telling on where her motives lie. She stops not when Uther apologizes, but when he mentions Tom. She's gotten off track, she realizes that, and for a moment it saves her.
Around the second season, Morgana discovers she has magic. Suddenly, she doesn't know who to trust. She confides in Gaius, who ignores her and pushes her away. When she talks to Merlin, Merlin is noncommittal, but at least he gives her a way to the druids. She feels at home and safe for the first time after a week of paranoia, and then she's being told she has to leave. Her self-centrism comes out, combined with her own trauma, and she's willing to let people die if she doesn't have to leave.
Her It's All About Me attitude is brought even further into light as she becomes insecure and paranoid about her magic. In a moment of desperation, she sells out Gaius to the witchfinder to keep him away from her. Despite the fact that she knows Merlin is there for her, she never goes to him. She isolates herself from all her friends, wondering whether or not they'd turn her in to Uther. When she rediscovers magical people, she's so distant she's willing to let all her former friends be casualties in the war to come. And then there's The Fires of Idirsholas.
Morgana, now focused completely on herself, literally falls in Morgause's arms when Morgause tells her how she can make things safe for her. She gets infected with a magical plague that begins to kill everyone. And then Merlin promises not to tell, and she tells him he's a good friend. Through this entire episode, it slowly begins to sink into her what she's done. She's chosen a complete stranger over a trusted friend. She's brought a plague to Camelot that is killing everyone she loves. She's about to snap out of it, and Merlin is forced to poison her. Every suspicion she had in the beginning is confirmed, the friend she could trust has betrayed her, and Morgause takes her away.
By the third season, we can see the results of Morgause's work. She's willing to kill Uther because he disowned her, never mind the fact that she's a bastard child and she's much better off as an adopted one. She may claim she's doing all for the sake of her own kind, but what does her reign in Camelot achieve? Nothing. Merlin even calls her out on it, pointing out that attacking the city with magic will just incite more hatred, and her response. "You don't have magic, how can you hope to understand?" is her new philosophy, laid out bare. No one except Morgause can understand her, and Morgause gets smashed into a wall by Merlin.
By the fourth season, she's lost Morgause. Everyone in her life is tools now, and her vendettas have gone from partially petty to entirely petty. She wants the throne of Camelot, and she wants Gwen off it. That's it. Nothing about her people at all. She might claim she wants the throne for her people, but all of her reign is making Camelot bow before her. She disposes of Agravaine, who has genuinely been a friend to her and has done essentially all her work for her. And moreover, her insistence on the destruction of Emrys. Does it ever cross her mind that he's a fellow sorcerer? That the future is not set in stone? No. He threatened her, and so he must be destroyed.
Fifth season to update when it finishes. Morgana has been held captive for two of the three year time gap, and has a motive shift from getting the throne to getting revenge on those who persecute magic. She's got a dragon at her side, has allied with someone, and continues her petty vendetta against Gwen.
MerlinWhile he never has a FaceHeel Turn, Merlin changes around quite a lot through the series.
First series, we get introduced to an idiot, careless, clumsy teenage boy with magic. He never thinks things through, which is established from the very first episode. Yeah, you moron, punch the knight who quite obviously has more combat training than you. You can totally accuse a knight of magic, because it's not like peasants don't tend to get listened to. And of course leaving your freaking poultice behind isn't going to cause trouble. Sure, he's a good guy, but just moronic.
Second season, cue Trauma Conga Line. Let's start off by almost getting Gaius killed. Then let's have Arthur go so far into Heroic BSoD that Merlin is forced to lie to him about sorcerers and set his feelings about them back even more to protect him. Then let's give him a girlfriend. Give him a way out of Camelot. And then let's kill her. And then let's have the Dragon manipulate him, Morgana betray him, and his father die while all the people in Camelot dying are his fault. Can you name one thing that could make this worse?
Third season, he's had a year to recover, and he's become much more cynical, as you'd expect. When Morgana turns, he doesn't angst about it like he would have last season. He tries to talk Morgana down, rather emotionally, but when she shows that's she's chosen her side, it's over for her. He admits to feeling sad about her, and regretful that she's become that way, but ultimately she's an opponent and he treats her as such. He nearly kills her in The Crystal Cave and regrets it, but only acquiesces because of everyone else suffering. He still feels for her, but it's largely Sympathy for the Devil. It's subtly done, but Merlin of Season 3 is a lot more cynical and much colder in the way he treats his enemies. note However, he isn't quite ready to kill in cold blood.
Fourth season, Merlin gets to shine in other areas than saving the kingdom from monsters over and over again as he becomes Arthur's unofficial advisor. Over and over again, he proves much smarter than Arthur, and what makes this different from other seasons is that, while he won't admit it, Arthur actually acknowledges it. The entire season is basically Arthur vs. Morgana, but it's more Merlin vs. Agravaine, as Arthur spends most of his time trying to find his footing, and Morgana spends her time concocting overly complicated and entirely petty plans. Agravaine and Merlin are the real opponents here, and it shows throughout the season. Merlin spends the entire season with a mole on the team, and gradually starts to realize why having a mole is a bad idea regardless of their good publicity.
And then the fourth season finale. It's Merlin at his darkest, and not just killing Agravaine, although that is a big part of it. Merlin has always had the potential to be a great Magnificent Bastard (manipulative, smart, knows things no-one else knows), and yet never really tried it. In this episode, he gets to go all out like he never has before. Good Is Not Soft, and boy is it chilling. Sure, it's not the most impressive bit of magic but he kills six men just by flicking his eyes. Morgan's right to call him The Dreaded. Then he plays Arthur and virtually all of Camelot like a well-tuned violin by carefully orchestrating the sword in the stone, sneaks into Morgana's bedroom and strips her of her powers, and then just smiles as she realizes she can do nothing. He controls everything in this episode, from the heroes to the villains, and all while making it look like none of it was his work.
Season 5 to update when it finishes. From what we've seen, he's stressed about Aithusa changing alliances, Mordred is the mole this time and Merlin can't just make him back down because his secret's at stake, he's flirting with the idea darker magic and he's become much more independent of his mentors.