I so enjoyed thinking about your question because it is very important to understand than the three actual witches (Alexandra Spofford, Jane Smart, and Sukie Rougemont) are truly protagonists while the one who causes conflict, namely Darryl Van Horne, is most definitely the antagonist.
While the three ladies are members of a coven and definitely witches, it is Darryl Van Horne who is more of an evil, demonic influence and, therefore, the antagonist. The degree of intensity in regards to the conflict they produce is astounding. Note how the antagonist, Darryl Van Horne, appears out of nowhere and immediately causes problems (or antagonism). He also divides the threesome of the three witches to no end. One is disgusted. One is charmed. One is not sure what to think. This division of the three protagonists is crucial to Darryl Van Horne being the antagonist. All of his thoughts and actions are meant to divide. He has orgies at a mansion. He collects art without respect for the artistic. He appears rich only to gratify his own greed. He is childish with a "playroom" meant for carnal pleasure. All of these things cause division between Alexandra Spofford, Jane Smart, and Sukie Rougemont as well as the town.
In conclusion, we can explore a bit more the meaning of the word "antagonist." It is a character that represents both opposition and division. In this regard, as well as all others, Darryl Van Horne fits the bill.