You probably didn't need us to tell you that Abner is the antagonist. He burns down people's barns and physically, verbally, and psychologically abuses his family. Abner plays a clear bad guy to Sarty's good guy. The entire story revolves around this relationship, and it's not until Sarty escapes from his father that we feel he has hope for a happy future.
De Spain is ultimately a victim of Abner's barn burning, but that doesn't mean that he is a good guy. We don't know enough about this character to ultimately judge his goodness. We do know that the fine he levies on Abner would have been paid for with Sarty's sweat. Since Sarty is our protagonist, that makes de Spain an antagonist. In the same way that the little boy in paragraphs 16 and 17 thinks Sarty is a barn burner because his father is one, de Spain considers Sarty his enemy because Abner is his enemy. Instead of being grateful to Sarty for warning him, de Spain almost runs Sarty down with his horse, and it seems to be on purpose.