I don't like May but I empathize with all of them
I just see it as another story of conformists vs. rebels - people pick up these roles depending on how well their personality fit in with the then social rules and culture.
I don't like May. Throughout this movie I tried to pierce through her upclassy veils and find that authentic May, but never succeeded. Every time she seems to initiate an "open conversation" with Newland, it ends up being just another "let's make this work" *beep*
The moment I was mostly pissed off - Newland accused May of thinking Ellen would be happier returning to Europe(effectively condemining Ellen to hell), but May seemed not be able to grapple with what Newland meant, simply dismissing it in a airy-breezy way "oh she shoulnd't have married overseas in the firstplace". That scene reveals that May can be cruel in her own way - but she won't realize she's being cruel, just as she won't realize she's never free.
On the other hand, I can see why May seems so annoying in our modern eyes.She was brought up under a conventional society, and her own personality seems to fit quite well with that system. That system did not tolerate finding one's authetic self - and May did not need to, because all those social rituals are a personification of her authentic self, ie. living in an idealised world and shutting out "anomalies" - that's the most comfortable way she'd like to live. This explains why she consistently remains so throughout her life.
But May's version is not necessarily the most comfortable way for Newland and Ellen. Their freedom-loving personalities don't fit in with the rigid way of living in the 1870s. In that social context Newland and Ellen are not allowed to express and develop their authentic-selves, and they did not find a way to break out with the upclass clique -- too much social pressure and they are not up for it.If they were born into our times in the 21 century, they would've been perfectly normal and welcomed here.
Still, May is the winner in her own time. She's kind of selfish for pulling Newland back into the "normal" world - but see how she has the upper hand here in the context of 1870s.