I’m wondering if I’m the only one who heard Arthur whisper ‘Merlin—' and instantly thought of the scene beforehand where Arthur told Merlin he never wanted him to change. Merlin has killed before, only for Arthur. But Arthur has never seen this side of Merlin, never known for him to be a killer. So while Merlin had never really in essence changed, to Arthur he had. The shock in his eyes when he watched Merlin thrust that sword into Morgana and the gasp that accompanied it. In his final moments of life, he realized all that Merlin had done, had sacrificed, all for him. Only for him.
"Only for you, Arthur."
Merlin, to Arthur, had changed. Only for him.