I watch her as she sleeps. I change her in her dreams. She will be mine so sleep now, my precious Elsa. <<< I imagine Jack possessed by Pitch. I don't ship Jelsa. Or Pilsa. But I imagine Pitch wanting Elsa so he can rule the kingdom through her.
So, I saw this and thought 'Jack Frost'. Then I really saw the dark side of his face, and I thought 'Is this supposed to show Jack in an alternate world where, instead of shooting Sandy, Pitch got Jack? And instead of "dying" Jack's mind was taken over by Pitch, and Pitch used Jack's immense power to destroy all the Guardians and cover the world with fear.