re: Boardwalk Empire Series Finale (Season 5, Ep 8 "Eldorado")

Shock maybe. The traumatic experience of being in that place was getting to her and mentally breaking her down.

Gillian was not crazy when she went to the institution. She was sent there as part of a plea in which she plead temporary insanity as a defense in order to avoid jail. She was hoping that she would one day be declared "sane" and released. In her meeting with the doctor he dashed all hopes of that when he said he could never be sure if anybody who was insane could ever be completely cured, that the insanity was hiding away in their bodies (ovaries, etc) and would have to be physically cut out. She was never getting out of there and would spend the rest of her days tormented as part of some arcane experiments until her death.

Frankly, I'd rather be in jail than in an insane asylum.