Could it be that Subaru himself killed Emilia instead of the witch? I mean his Miasma gets stronger whenever he dies. Before it couldn't manifest outside of himself. and only squeeze a heart. The closest one. His own. Now he accidentally summoned those hands himself and focused on Emilia. Sure he wanted to tell her but thanks to that he focused on her and this his own Unseen Hands killed her. My crazy little theory anyway.