HARRY POTTER FANS!!!! HELP!!! theres sdomething i dont understand!!!?

It explains it in the movie that lupin was actually afriad that it would have turned into voldemort and when it turned into a bogart actually assumes the ability of a dementor which affects harry greatly. In a scene later in the movie you see him learning the patronus against a bogart and he still faint. As to why he dismissed the class I'm not to entirely sure he wanted the students to start asking questions as to why he stopped harry and when he did why his fear was the moon. Kind of an awkward situation for a teacher/werewolf.