This and the next few verses present the first ‘unclean spirit’ scene in Mark. In this context, the Roman conceit of the superiority of their culture led them to assume the only thing leading to rebellion was evil spirits or insanity. The fellow Jew with the ‘unclean spirit’ wanted rebellion: it was his irrational, unclean spirit that caused the need for rebellion.
The transition from this scene into the next shows that the man with the unclean spirit was Rebel Jesus. This is the first confrontation between the two. The setup had Anointed Jesus teaching (i.e. having authority) within the synagogue, and Rebel Jesus is “some shouty guy”.