I have discovered that I don't like when fanfics have Arthur react badly to a magic reveal and then continue to have him be insensitive/rude/mean/discriminatory towards Merlin afterwards.
Of course, Arthur canonically reacts negatively to Merlin's magic reveal, but he comes around fairly quickly and the real issue was the deception rather than the magic.
Sometimes I'll read a fic and Arthur will be incredibly stubborn about magic being bad to the point where it's out of character. He had lost his mother to magic, but he still wanted to see the good in it when Uther was dying. Even after Uther died, he wanted to see the good in magic to save Mordred. He's always tried to consider the alternative.
Season 2, Episode 08 - "The Sins of the Father"
"What if my father's attitude towards magic is wrong? Perhaps it's not as simple as he would have us believe. Surely not everyone who practices magic can be evil."
Season 5, Episode 05- "The Disir"
"Perhaps my father was wrong, perhaps the old ways aren't as evil as we thought."
If it was the magic that bothered him in canon, he would've reacted far different. For example:
Arthur just watched Merlin (possibly) kill those two men. He watched his best friend and the person he trusted most use magic for violence; this is what magic is falsely associated with, but Merlin potentially confirms it in front of Arthur.
If the problem was the magic, he probably would've said something more along the lines of Merlin being a bad person or a monster. Instead he's far more focused on the fact that Merlin lied to him.
I don't mind if characters in fanfics are out of character sometimes, but I guess in this case, since it leads to Arthur being horrible to Merlin, it bothers me. It's an added reason to make him be cruel to him when there's plenty of reason there for a negative reaction already.
Maybe I'm just a snob or overly sensitive, but I guess an inaccurate portrayal of Arthur in magic reveal situations gets under my skin for some reason.