I’m not able to find page numbers (pdf websites are allergic to them apparently) so in their place I’m using the format of (author, chapter number)
In chapter 15 of Lady Audrey’s Secret when Lady Audrey is attempting to make Sir Michael believe Robert Audrey has gone mad, she spends a decent chunk of text going on about how many people may know they are mad and hide it or keep it under control, but they also might go on to snap and kill someone. She goes into quite a bit of detail to saying things like “the frightful, passionate, hungry craving for violence and horror” and other oddly specific and disturbing details (Braddon, ch 15).
It is even called out earlier with “She spoke as if she had been discussing a subject that she had often heard discussed before” that she might be discussing madness beyond just Robert Audrey (Braddon, ch 15). Just maybe, she is referring to herself. Her past with others who believed her mad or overhearing others talk about madness around her and Lady Audrey picking up on similarities to her mental state. This could be Lady Audrey getting caught up in using an excuse so close to her own heart, or perhaps even a warning for Sir Michael.
Either way the detail she goes into and the specific example of murder are far to close to previous implications in the book that Lady Audrey may have murdered some people for mere coincidence. Even in the extremely unlikely event this is all a red herring and Lady Audrey is completely innocent, her fascination with madness and apparent research on the topic are worrying signs for her mental state.