I have a feeling that Goddess is also a reincarnated person or someone entirely different from what the people of that world think she is. Last time she had a line speaking, it sounded like she was having "ecchi time" with the Demon Lord rather than fighting him.
They've been trying to assassinate him behind his parents' and grandparents' backs for years, thinking he was easy prey. Never did they think he would fight back against them.