I mean personally,i get your point however well i can see also see how Morgana might have also hold Arthur accountable (which i feel he is in a minor way) as he knowingly accepted the only place of power that could have granted Morgana to take back all that was her,but indeed the action she took to counter that was ultimately a vile with no justification..the same one done to her mother but it was one done by a person broken by injustices done to her..and at that point,i don't think she was thinking from her moral compess but utter desperation and extreme grief even a bit of insanity,in a messed up it is hard not to sympathise with her going to such extreme measures,as she was a child who not only lost her parents but witnessed destruction of all she hold dear