I suspect it was (spoilers) . . (I hope I'm wrong but) . . . If you watch the episode, Kevin goes looking for Fangs and sees him chatting with Midge, ten minutes before curtain, and therefore less than ten minutes before she dies. The both of them seem greatly amused and not at all aware that something terrible is about to happen.

So who do I think killed Midge ? . . . Moose! . . . I seem to recall Moose striding backstage looking extremely angry. . . . Here is what I think happened. Fangs outed Moose to Midge. Rather than getting upset, Midge probably thought that was hilarious, and couldn't wait to tell Moose she knew. She was either cool with it, or that it explained a lot of problems they were having as a couple.

What that she failed to take into account was Moose's level of internalized homophobia. She probably teased him about being queer, and he struck her in a blind rage, killing her instantly, and then he had to stage it as a Black Hood killing, using Ethyl's threats a cover.