Interestingly, very few fans have picked the mayor as a Black Hood suspect. Obviously, Mayor McCoy has the connections and the knowledge to pick off people who are causing problems in Riverdale. You could even argue that her moves come from alliances with other families. Perhaps the attack on Fred Andrews was her way of earning the favor of Hiram Lodge, a powerful businessman and crucial ally to have. And you could tie Moose and Midge and Mr. Phillips up in the Jingle Jangle industry, which is clearly a thorn in McCoy's side.
This solution would explain why McCoy's affair with Keller has gone completely unnoticed and unaddressed by the Black Hood, even though Keller and McCoy would surely be considered sinners. As for Miss Grundy? She could have tarnished Riverdale's reputation for good by sleeping with a student. What better way to punish her than to drop her ass in Greendale and then have her murdered?
All things considered, it would be pretty hard to explain Midge's murder, but McCoy could be cleaning up a previous mess. Obviously she's not the person behind the mask, but she's got that town money to help her get sh*t done.
