What happens in the attic between Lissa and Christian in Chapter 9?

Christian explains to Lissa that he thinks vampire magic should be used in combat against the Strigoi. He shows her a type of non-burning fire that he made for her. Christian also points out that Lissa uses forbidden magic. Christian suspects that Lissa must have used compulsion while she was away, and tells her that he as seen her use it at school. Lissa tells Christian that Rose doesn't trust him and thinks that he might have killed the fox. He denies this and offers to hurt whoever did.