@Notesha
It was to steal her skill, the MC concludes that the women most likely have some sort of skill that helps her to manipulate the hero, so he decides to rape her to steal it.
Dont know why this slipped my mind the first time I read it.. but she wouldnt give him any information about the demon king.. Its not that she didnt have information, she just refused to tell him O_O
When fighting a difficult opponent. information about them is vital.