*deep inhale* it’s the way you can watch the emotional equivalent of Arthur getting punched in the gut when he finds out about Merlin’s magic. It’s the way you watch him grasp at straws “You aren’t a sorcerer, I would know”, “This is stupid why are you saying this” not because he fears magic anymore - he commends the sorcerer for winning the battle seconds before - but because it makes sense and if it’s true, it means the one person he thought would never betray him has. It means the canon one person he truly trusts never trusted him back. And you just watch his heart break.
It’s the way you know Arthur would’ve screamed at Merlin, fought him, demanded to see the full extent of his magic if he was healthy, but he’s dying and like it or not his life is in Merlin’s hands. It’s not about the magic, not at this point, it’s about the lying.
It’s the way Arthur makes the same punched-in-the-gut face when he watches Morgana announce that she’s his sister, when he sees Agravaine fighting alongside Morgana, when he finds Gwen and Lancelot. And Merlin, the one person who has stood by him for all of this, was lying too.
It’s the way when Arthur says “You’ve lied to me all this time.” it’s not only a revelation about Merlin but a revelation about himself. The realization that his victories were not his own. The knowledge that everything he prides himself on, everything he has done for his kingdom, was more Merlin’s accomplishment than his. Arthur already has crippling self-doubt, and to realize he has just been a pawn in everyone’s long-winded game including that of his very best friend is destroying him mentally and you watch every time it hits him that he has done nothing (he thinks) without the interference of another to ensure his victory. He’s being used as a pretty face to front the movements of everyone around him because he has always been more the King of Camelot than Arthur Pendragon.
It’s the “I trusted you” “I’m sorry” “I’m sorry too” and the way that Arthur blames himself for Merlin never trusting him. It’s the way “why are you doing this?” isn’t just about Merlin behaving like a servant but asking Merlin why he’d fight so hard to save the life of someone he never trusted.
It’s the way “I want you to always be you” is Arthur giving Merlin the vocal affirmation of trust that he thought was implicit all these years that Merlin never understood. It’s the vocal acceptance that friendly bullying never let on. It’s the “You may have lied to me but I still trust you with my life” that Merlin needs to hear.
It’s the way “thank you” and Bradley’s mouthed “I love you” are Arthur’s final ways to trying to show Merlin that he truly did care about Merlin in their time together and that while the way he was raised forbid him from showing affection, he did what he could in the ways he knew how.
It’s the way they could’ve done so much more, been so much happier, if they had just confided in each other, rather than be convinced that they had to do everything alone like the depressed bastards they are.
It’s the tragedy of Arthur Pendragon for me chief.