User blog comment:B214/Question and Answers 6/@comment-3285616-20171119130645/@comment-120.188.37.253-20171125185100

It was actually stated in Vol 15 when Akeno told Issei and co about her past

Here is her quote

"I hate men. I really hate adults. They were only worried about how they will appear to others, and didn’t forgive my mother at the end……! They also see me as a disgusting creature."