Theory: Spencer says the girl falling in the bottom picture is Marion Cavanaugh (Toby’s mother). The picture before, it looks like the same girl with same dress running after a monster carrying a boy. I think the girl running is Marion, the boy is Toby and the devil is Radley/Mrs Dilaurentis not allowing them to see her son.
I think Marion blamed Mrs D for not allowing her to see Toby. (Maybe Mrs D was convinced that Marion was getting sicker? Or not allowing her to see her family as a punishment) In S04E04, Toby talks to Marion’s old doctor. He said “She hated being away from you, but there were forces that were just too much for her”
Bethany must have been friends with Marion and was drawing about her friend’s life. When she witnessed the injustice of how they treated Marion, followed by her “accidental” death that they covered up, she wanted to take revenge for her. Maybe Bethany secretly witnessed Mrs D push Marion. So then Bethany started calling her a liar because Mrs D just covered up Marion’s death by saying it was a suicide, which enraged Bethany. Mrs. D brushed it off as “just a mental patient acting crazy.” I think Mrs D knew Bethany’s true hate for her because she got a call, possibly from Radley the night Ali was hit, saying that a patient that hated her broke out.
When Mrs. D witnessed Ali get hit by Bethany, she knew she couldn’t call the police. For the longest time, I thought she was protecting the person that hit Ali, but in this instance, I think she was protecting herself! She knew if she told police that a patient at Radley killed her daughter, they would dig deeper into why Bethany would target Mrs D. They would then find out that she had a hand in Marion’s death.
So in conclusion, Marion was denied her son. Mrs D pushed Marion to her death. Bethany, being a great best friend to Marion, held a grudge against Mrs. Dilaurentis. She decided to kill Mrs D’s daughter. Essentially taking her child away from her like she did to Marion.