[spoiler] I read this theory that says that the whole movie takes place in Kate's head, like a re-imagining of something tragic that happened to her. Her tattoo is a snake eating itself, which is a symbolism of Kate's madness. [/spoiler]

[spoiler] The article explains that Kate IS Ms Jessel (the former nanny). She worked at Bly Manor and was charged to raise Miles and Flora, but she was afraid of Quint who was a creepy pervert and one night, when she tried to escape, he sexually assaulted her. She fled the manor and abandoned the children, and she went insane because of the trauma and her regret over leaving the children. She was taken to an asylum, and the movie is Kate in her head living out the life that she should have had, the path that she should have taken in which she saved the children and escaped with them from the clutches of Quint. [/spoiler]

[spoiler] There are a lot of things that prove that Kate is Ms Jessel. First, Kate always sees Ms Jessel through a reflective surface (window, mirror, etc...). In Kate's fantasy, things are perfect and Quint is dead, BUT, due to her subconscious, Quint keeps creeping in at several points as a ghost and through Miles who is actually a manifestation of Quint. Miles wears the same clothes as Quint and rides a black horse just like him. Besides, the first time Kate sees the ghost of Quint is when he is walking into Miles' bedroom. Miles also almost forces himself on Kate at one point and kisses her, which would be a symbolism of the sexual assault that Quint carried out on Jessel. At one point in the movie, Kate believes she is being attacked by the ghostly hands of Quint, which could be a flashback of her past sexual assault. [/spoiler]

[spoiler] At the end of the movie, she returns to reality where she is back at the asylum, and when her mother or rather Kate turns around, she sees her own face, and this is why she screams. [/spoiler]