twist/turn the knife (in the wound) idiom
揭人伤疤;往伤口上撒盐
to make someone who is annoyed, worried, or upset feel even worse
Just to turn the knife a little, he told me he'd seen my old girlfriend with her new man. 只是为了让我不好受，他告诉我他看见我的前女友和新任男友在一起。