...I don't quite know what to say... good, bad? yes, no? I don't know, it isn't like I haven't seen worse in real life, and in those situations generally at least one person isn't happy with the relationship, if they love each other and are both happy who am I to judge?
My interpretation is that the bully needs to hide her love for her victim by bullying her. Part of me thinks, 'Aww... they love each other,' but another part of me thinks 'Holy crap, there's no excuse! Bullying someone is not a sign of your "ardent love"; it's abuse.' I don't know what to think. It would have been nice if there was a conclusion when the bully turned around and stood up for her girlfriend.
I feel this story is far simpler than it lets on. My understanding is just that the girls are in a relationship and do love each other, but Bully Girl has to keep up her tough girl persona around school ("The little garden we dwelt in was very childish and much too superficial") so they have to keep it a secret. Chiaki the victim understands this and loves her anyway.
Not sure it deliberately has anything to do with S&M since Bully seems genuinely apologetic about the treatment. It's still really fucked up but I sort of get where its coming from.
I feel like it's a little more nuanced than that, to be honest. I think the Bully girl likes bullying people(since she seems like she's enjoying herself on the top left of page 2), but she also hates that she does it(as seen with how regretful she looks and how she starts crying at the end). I think it's rather similar to someone who has a terrible addiction(like gambling, smoking, cheating, etc) that feels really good when their in the act, but they always feel regretful when it's over.
It's like a "I know this is bad, but I just can't stop" kinda thing, and I feel like the dialogue proves this with phrases like, "her love is so warped// and hopelessly clumsy" and the fact that the protagonist girl sees this side of the Bully Girl as a part of who she is. I still don't think it's healthy, though