so anyone really thought about the meaning of the name of the final quest? The Cure for Insomnia... the cure of a person that can't sleep, Noctis killing Ardyn is basically him putting an end to his immortality... Ardyn can finally sleep.
It’s a very poetic way of looking at it but that’s how I saw it as well, especially if you’re of the thought that Ardyn is the underlying dark horse/ central character for the game’s story - not Noctis.