Could we find the theme of insanity in Doctor Jekyll and Mr Hide?

I am an English teacher in a high school. We are studying Robert Louis Stevenson because the students have to talk about the theme of insanity at the exam of the end of last schoolyear. However the main theme in this story is most of all duality so I don't know how to find details for insanity. Although Mr Hide represents the dark side of anyone of us I can't say this is insane. Maybe because Mr Hide behaves as a mad since he kills without any repent and consideration of people? I would like to have some advice

Thank you very much indeed

Maria