I'll be the first to admit that Moose isn't a super strong candidate, but there are a couple of worrying things in the musical episode. As you probably noticed, Moose is very upset just before the show starts. The easiest inference we can make is that Midge has been cheating on Moose with Fangs, and Moose has found out. Would that drive Moose to murder Midge? Slim chance, but it's a possibility. And he does look pretty heated backstage.
For this to work, Moose would have to be a new Black Hood, since he has previously been a victim on the masked madman. But, hey, never say never. Maybe Moose is taking a dark turn.
