I know exactly which fic this one was inspired by. The ninth chapter in Ultra-Geek's One Hundred collection of drabbles. Because I love protective Arthur. And I guess also chapter 86 of the same story, because while I know Gwaine has found respect for Arthur, I know he would totally do something like that (or this).
"Listen to me very carefully, Sir Gwaine."
The knight leaned forward as the prince spoke softly.
"I am about to arrest Merlin."
"Merlin?" Gwaine exclaimed, outraged. "On what charge?"
"That of sorcery."
"He will be found guilty and he will be sentenced to death by beheading, to be carried out at dawn."
Gwaine reached for his sword, though not sure of what exactly he planned to do with it.
"Unfortunately," Arthur continued calmly, ignoring the movement. "I will not be surprised if, using these evil powers he has, he manages to escape from the dungeon by morning. He may even be able to take a knight hostage. That would not surprise me either."
Gwaine's fingers relaxed slightly, but his hand did not make its way back to its most natural position.
"Yes," he agreed slowly. "That does seem likely. And unfortunate."
"But how can you be certain he will be convicted?"
"He performed magic at the banquet in front of the whole court. My father is demanding his head.
"Ah." Gwaine turned to leave, knowing he had to get started if this was to work.
"Hold on one moment, Sir Gwaine."
The knight turned again to face the man he would gladly call king one day.
"That knight might tell Merlin that I will never forgive him for this betrayal."
Gwaine stood straighter, registering Arthur's level voice that didn't sound the least bit angry.
"Or inform him that I will kill him myself if I ever see him again. He will receive no mercy."
"Yes, he probably would."
Gwaine made himself scarce and was nowhere to be seen when Arthur led Merlin to the dungeon, face cold and distant.
But no one was around when the drunk-prone knight and Merlin snuck away after dark.
Once they stopped for camp, Gwaine began to speak, weaving tales of a livid, murderous prince who didn't care what they had been in the past but hated him just because he performed magic.
And though it killed him to see his friend so upset, Gwaine agreed with Arthur, and did what he had to do.