Arnold Friend is an antagonist, yes. He opposes the protagonist, who is Connie. Antagonists are not always evil and villainous, though, but in this case, he is evil and has the worst of intentions. Arnold is a strange man who certainly could be the devil in disguise, according to eNotes:
Many critics have suggested that Arnold Friend is the devil in disguise. He has trouble balancing on his small feet—hooves?—and the make-up on his face makes him look younger than he really his.
Arnold is a dangerous, murderous antagonist in this story, and it is easy to immediately cast him in that role because of his actions and words.