I've got a theory about Mary's behaviour. She's been acting quite similar to how she was in series 1, except without her sympathetic points. When she made bitchy comments back then you could kind of see that it was often the result of her fustration and unhappiness. But now she is just as rude and sarcastic but seems to have everything she wants. And compared to Edith, who is genuinely happy I think, Mary looks kind of pathetic. It feels like she is trying to get a rise out of Edith, while Edith either rolls her eyes or just makes a sarcastic comment back that usually shuts Mary up (like last episode).
So I think Mary is unhappy. I also think Mary is trying to recreate the circumstances in which she met Matthew in order to meet another Matthew. She wants to be the interesting and beautiful debutante again, with men throwing themselves at her feet. And she is trying to bring back her old relationship with Edith in order to make herself look better. She wants Edith to go back to being the pathetic spinster. But Edith isn't playing ball and has become confident and happy.
I think it all links into the theme of Mary trying to hold onto the past. Whether it be Downton or who she was when Matthew was around.
I have to admit, I'm not sure I hate Mary as much as I now pity her. She just seems to be really pathetic.