Elijah was an angel. Angels and demons were enemies, and grew to hate each other over years and years. Angels sworn to protect humans, demons sworn to torment humans. Elijah was sent down to kill a demon, as he was told that there was a demon causing trouble. But when Elijah actually saw the demon, he immediately felt conflicted.

You were a child. A demon, sure, but only a child. Elijah has always had a soft spot for children, and you were perfect for that. Elijah didn't know what to do.