She would probably be very indignant. Hermione is known for her strong sense of justice, so she would likely demand an explanation and try to prove her innocence.
She might be shocked at first. Given that in the original story Morgana has her own complex relationship with magic, seeing Merlin's magic could stir a mix of emotions in her, like envy or a sense of relief that she's not alone in her magical abilities.
Dazai is not based on a true story. Usually, characters like Dazai are formed through the author's imagination and artistic expression to create an engaging narrative for the readers.