"…she knew well how all she had done would be perceived. still, she’d do it over again if it gave her people a fighting chance…she had done what she believed to be right for her family and her people, to protect those she loved."
from a camelotian perspective, kara’s treason is very clear-cut and there is no other possible way her actions could be perceived. and while kara is under no illusions as to how her actions will be construed — she is neither ignorant nor naïve enough not to know this (or to believe that her intentions will matter at all to those that accuse her) — for her, what she has done is not so simply decried as treason. her intentions are to help those important to her; she would very much like to, she is not plotting the downfall of camelot, and she does not actually mean arthur any direct harm. could her actions be considered a threat to arthur and his rule? of course. but in truth, kara thinks only of her loyalty to her family, helping those in need and providing an advantage to her kingdom.
additionally, though kara could never be so easily made to do anything she did not want to do, she makes none of these choices to act in a vacuum. yes, her personal hatred towards arthur for the deaths of her immediate family plays some role, but she is also guided by a deep loyalty and love towards king lot and queen morgawse, as well as her husband, and by the fact that she is the future queen of the gododdin. furthermore, she is influenced by the fact that she has prior involvement with the rebel factions within lloegyr, and by an iron-willed conviction that it is not a crime to be pagan or practice magic.
so if she’s not scheming the murder of king arthur, what does she do? firstly, she picks up where left off during her self-imposed exile, aiding pagan refugees and witches to evade the camelotian authorities and escape the kingdom to safety. secondly, she acts as something of a spy and informant, passing information back to lot and morgawse, and occasionally the other kings of alba. importantly, medrod is entirely aware of kara’s actions, as are their brothers (though perhaps to a lesser extent their wives, if only for their own protection). and like her, he sees her actions more in terms of aiding alba, rather than undermining camelot. in fact, much of the information that kara passes back to alba, is initially gathered by medrod and his brothers and told to kara knowing what she will do with it.
this is not to suggest that arthur is then naïve for entrusting any information to medrod and his brothers, but I do think there is some oversight in arthur’s thinking that allows him to put more faith in the orkney brothers than is warranted. his fosterage of the four younger brothers was significant, and culturally arthur could not be blamed for imagining that the bond between foster-father and foster-son (let alone that between uncle and nephew) was strong enough to prevent betrayal, nor for believing this is some way extended to medrod, who certainly capitalised on the ideas of kinship between them since coming to camelot. arthur may have also truly believed that his victory in the second alban revolt and lot’s subsequent preoccupation with the saxon encroachment had truly brought the conflict between their two kingdoms to an end. it’s understandable then for arthur to have put great trust in his nephews, even if that trust was ultimately misplaced, and also explains his unwillingness to believe medrod’s utter complicity in kara’s treason.