I think we should think of this more simply as even if she does beat the demon lord, she will eventually be one. I take flammes line here quite literally as she is essentially still a warmonger. When her counterpart is gone she will become the one who brings death. I mean, technically shes a walking nukes that can do whatever the hell she wants, she also willingly bestows another nukes to another creature that piques her interest. Who knows what kind of hellscape she may make. Not to mention the aftermath of their fight might even leveled up the entire continent.

Perhaps the lesson we can take is its not JUST about overpowering the other.