Callisto was still a worried wreck. This was a side of her that Merlin hadn't seen before.
Her public image was most likely that of a hot-headed violent princess who wouldn't hesitate to shoot a noble in public, but Merlin had seen her softer side that was used almost exclusively for those in the rangers guild.
And although Merlin had seen her emotions when faced with the death of members in her family, specifically referring to the guild members, and not the royalty she found herself related to by blood. He hadn't yet seen her reaction when she almost accidentally killed one of them right in front of her.
It was kind of morbid to think about, but Merlin thought her reaction was kind of cute, even though he was the one who had almost died. It was mostly due to her contrasting behaviour when compared to the time she had almost murdered Baron Doolmet in the same manner.