Arthur is not “acting in self-defense” by executing revolutionaries. The revolutionaries are the ones acting in self-defense. The governmental authority is the aggressor, whereas the oppressed group is the one victimized. Abuse (in this case, social abuse) is about power imbalance, and the power is ultimately all Arthur’s. Magic or not, Kara carries no social power as an oppressed woman. Arthur, on the other hand, decides what will be law and chooses to maintain the same unjust, genocidal rulings as his father before him. Kara acts in self-defense and in defense of others, as a martyr, in her assassination attempt against Arthur, who is a tyrant against her people.
Arthur is prideful in the most significant sense: he cannot and will not admit to his wrongdoings against magical peoples, and in fact never meaningfully develops beyond his initial classist evaluations of the people in his life. He is only willing to consider lifting the ban on magic so long as he has a personal stake in it, never once considering that what may be maligned to the monarchy is exactly what would benefit the people he is responsible for. To do this, he would need to truly comprehend that he is not more worthy than any other person of insurmountable power and hoarded wealth. As for the latter, when he deems a commoner worthy, he simply promotes them to a higher status. This implies that all who are deemed commoners are seen as less worthy by his estimation. Merlin remains a commoner despite his invaluable contributions because Arthur lacks true respect for him as an intelligent, dignified individual.
Uther is frequently cited as an excuse for Arthur’s behavior, but if Arthur’s behavior needs excusing by this method, then you are deliberately omitting the fact that he is undeniably comparable to Uther in his continued oppression of those he deems inferior to him based on class, religion/ethnicity, and gender. You unintentionally acknowledge that he maintains the same systems that Uther put into place, but with slightly less aggression. This isn’t “not bad with Uther as a father,” this is a direct result of glorifying Uther’s tyrannical rulings out of monarchist pride.
Most interestingly, you say that you can’t even call Merlin “good,” as you claim this is a misconception based upon his extreme devotion to Arthur. This is quite an assumption, as it is this very devotion to Arthur that threatens his moral standing. It is even more telling that you hold Arthur to such a low standard by comparison to Merlin. Arthur’s moral ambiguity can surely be excused by the fact that he was raised to be immoral, yet Merlin was raised under the same social norms, only as a peasant farmer rather than royalty. Arthur is coddled to remove his accountability in his own actions and beliefs, while Merlin’s autonomy is acknowledged only to convict him. Merlin’s comparative lack of autonomy compared to Arthur, who holds the highest social and political power in the land, is methodically omitted.
Morals are not secondary to Merlin — they are in fact his primary quandary. Because his morals are often challenged and he feels, at times, forced to compromise on morals he would rather not, some fans are under the illusion that he simply does not struggle with morality and is always willing to forgo his morals to achieve his end-goals. In reality, these events show that the exact opposite is true: Merlin’s primary struggle is in maintaining his morals when the destiny he has been landed in challenges the breadth of his autonomy. Whenever possible, he attempts to find another way — such as lifting the fever curse on Camelot and saving Morgana’s life, or saving Camelot and sparing Kilgharrah, when these were set as mutually exclusive goals — but as a poor commoner and servant who is forced into hiding as part of an oppressed class suffering genocide, he is wholly unable to wield the same social and political power that Arthur does. That is the goal of kingship: to withhold power from other people and permit it to oneself.
Merlin’s “purity” comes from an absence of moral “contaminants,” these being arrogance, selfishness, cruelty, and indifference, among other things. Arthur did not kill the unicorn for any moral failing of the unicorn — he killed it because it is a magical creature and killing it would boost his reputation. The combined factors of pride and cruelty on display disqualify Arthur from the same comparison to the unicorn that Merlin is subject to, especially given Merlin’s status as an oppressed creature of magic under Arthur’s authority. The unicorn represents Merlin because both would be killed for their mere existence, not any true crime they committed, and both show a certain moral purity in their absence of cruelty.
Case in point, monarchism is wrong, Arthur’s actions and beliefs cannot be excused no matter how you tilt the tables in his favor, and if Merlin were given the same tests as Arthur was, he would have made the decisions that earn the title “pure of heart” because he does not possess the same flaws as Arthur does.