When Ib and Mary go find something that will break the vines and find the palette knife, Mary takes it. Mary told that picture to split them up and by splitting Garry and Ib apart, Ib wouldn’t get hurt by Garry. So when they exited the red eyes room, the flower picture split them up. She thought he was going to hurt Ib, her new friend and Mary wanted to protect her. In Mary’s point of view, Garry was a dangerous man, carrying a lighter and destroying art.

That’s why she didn’t listen to him and payed more attention to Ib. Mary already knows who Garry is, when he destroyed the head. Skipping to where they meet Mary, now this is where things get interesting. I believe that one of the other art pieces told her that Garry destroyed the head. Mary already knows that two people are in here and is on her way to meet them, when she senses that someone hurt her friend (the mannequin head). Now lets go to Mary while you select the option to destroy the head. In the game if you let Garry damage the art, you might get the bad ending. Now lets skip to where Garry and Ib meet. Not all of the paintings or art tried to kill Ib, but they helped her. Lets skip to where Ib and Garry end up in the fabricated gallery. My theory is that Mary was indeed good and was just trying to protect Ib, like Garry. In the game Ib, Mary was the bad guy that tried to kill Garry and be with Ib, but what if that wasn’t the truth.

Ib the game mary art