Am I supposed to be on any of these characters sides?
I don't think you are meant to, but it's an option nonetheless. It certainly adds to the feeling of how good this manga is.
It's probably the one instance of Reversal tag done best. Airi is not portrayed either as profoundly evil, or as a bully who can be justified. She is just an asshole who was full of herself or rather insensitive and indifferent. Cowardly to boot. And how wonderfully their relationship evolved. From consented bullying to treating Hina like trash to absolute dependence on her. Yeah, that feels impactful. It's funny how Airi was cheerfully at the start with her perversions and how they became her nightmare fuel later on.
At that point, I don't even know anymore if a happy ending is possible. Even if yes, do I want these people to be happy? Probably something along the lines of: Megane gets exposed, everyone blame her and make her a punching bag. The other two friends of Airi contact her and make up with her. At the same time Hine feels like they can't live on like this and sacrifices her relationship with Airi, by telling her all about her schemes in order to convince her to go back. Somehow after all of this they regain their social life and their relationship is not completely broken.
But it still feels soooooo much out of place. Only Ichika will be happy in this Mayhem. If she can survive the yandere's rage....