I have also thought on that point. But of you think of it another way, it was essentially a dogfight over his soul. The evil spirit/devil wanted his pure soul and Elzay wanted to save it. The spirit put him through pain to corrupt him and Elzay would put herself forward to heal him. Anytime the spirit almost won his soul, Elzay would take it back by giving her life in exchange. Elzay wasn't expecting to be saved, The goddess was the one who refused to give up on the girl she blessed. All in all it ends up one big never ending dogfight where noone is willing to concede until Aseph finally found the will to purify himself on his own by sacrificing his own life to rid himself of the spirit. The real question is why hasn't he ever sacrificed himself for her before?
Wouldn't the dogfight never even happen if Zay didn't start the chain of events? Aseph would've died in the fire if Zay didn't sacrifice her life, and if he died then and there the evil spirit wouldn't want his soul. ( ╹▽╹ )
I mean that's true, but she would have to be one heartless person to find out the kid's fate is to be killed and swallowed by the devil and do nothing. Tho I do think it's a bit of a stretch to say a 3 year old was completely ready to give up her life with no fear simply because he was her soul mate. Would a 3 year old even understand the concept of what she felt?
If Zay didn't push for Aseph to be saved as a 5 year old, he would've died there and he wouldn't have suffered countless awful things over and over. She did "give him his life" technically, but his life was fucked after that, honestly. Not to mention, there were zero assurance that the goddess would take pity on her again. For me, that partannoying.