Tomatsu Haruka, you are now my favorite VA. That was one hell of performance.
Not only are they out of practice, they are terrible at fighting. The military of the central nations is stuffed with the sons of nobles, used to only the cushiest of guard duty. When faced with the prospect of combat, they tend to desert.
That wasn't Maou. That was the Spirit of Light herself. She was lamenting that her existence is what led to the world being torn asunder. Her name used for Crusades, her name used to begin the war between humans and the demons in the first place. She sees the birth and death of everyone on all sides, and it hurts her. She feels pain from the death of each one. It pains her to see them using her as a reason to kill each other, when her true desire is the protection and well being of all.
The segment was included to make it clear that coming conflict is not a religious war. It is a political one. The Central Church is misusing the Spirit of Light's name to enact their own will on the people. Lady Knight's view of the Spirit is the correct one, she wants peace and prosperity.
The flashbacks of Maou were a bit misleading. Or perhaps she is influencing Maou a bit, allowing her to override the will of destruction from the preceding demon lords.