@Blusasuke
I know that manga and I have read it too but in that story that girl is already corrupted to being with she herself throw him into the dungeon in order to save herself ...but when she met reality she become went more and more into path of darkness that she thought to kill everyone but meet her own death by the hand of mc..!
its annoying but if your in that situation you will feel it's better to die then live in that corrupted world with all that sin she had done....!