Why did Constance Langdon specifically request Violet eat the poisoned cupcake. Well, she probably wasn't too keen on Violet hanging out with Tate in the beginning... she is rather possessive of her good son. This was right after a phone call in which Ben told her that he was going to stop treating Tate because of his inappropriateness towards Violet.
A lot of reviewers didn't get why Vivien would eat the brain and pancreas. Other than being an allusion to a famous horror film it was probably for the anti-christ and the anti-christ baby inside her was craving it so she ate it too.
The Langdon family inclues Addy Langdon, who has Down Syndrome. Down Syndrome was identified by John Langdon Down.
All the ghosts pretty much owe their death to Constance (at least the ghosts from 80s-present day and the entire Harmon family, due to Tate and Michael, her Anti-Christ grandson. So why haven't ANY of them so much as gotten violent with her or simply tried to terrify her? They're afraid they will kill her. She is so awful that not even literal monsters like the Infantata want her around any longer than she has to be.
Tate's constant references to Kurt Cobain and grunge-style clothes make more sense after the revelation that he has been dead since 1994 and therefore his cultural knowledge would be pretty rooted in the early '90s culture.
During her talk with Ghost Vivien in "Afterbirth", Moira brings up a point about how Nora was unstable and her being a mother would have only worsened it. So, what would have happened if her and Charles's baby wasn't kidnapped?
Probably one more Tate in the house.
In the beginning of the season, Vivien has just had a miscarriage and it's what propels the whole plot forward. It's the first dip into the theme of children and the world of death and living. The whole series is wrapped around the idea of 'child death' (you can even see so in the credits) and loss of children, resurrection of children. It begins with a miscarriage and ends with one when Vivien gives birth to twins and the anti-christ lives and goes into the real world, with Constance, and the other dies and lives with Vivien, finally giving her a child... but in order to have that baby she had to give her own life as well. Which, Nora also did, she killed the Infantata but then herself and her husband. The Murder House was a place of abortions under her husband, Charles. The whole show is about miscarriage - a place between living/creation and death, and the stillness-trap in between. The anti-christ is how the show ends and it is made of life and death.
and metaphorically these ghosts all cannot pass into the other world and are trapped among the living, like miscarriages.
Tate is a sociopath right? Guess who he inherited it from? Constance. She has several red flags to being one too. First her murdering Moira and tormenting her as a ghost, second making Addie where a mask to hide her face during Halloween which ended up distorting her view of things around her, if Constance hadn't mourned her so much it could have even been seen as on purpose. she didn't watch her that night either. It's common for sociopaths to have multiple children and even a case study of a woman losing six to violence that she hadn't performed herself but stood and watched as they were killed (just to have more). Constance may not be a full sociopath but she definitely has some traces of being one.
Plus trying to poison Violet, not warning the Harmons more and defending Tate even after he shot up a whole school.
In the final scene Constance gives a speech about how all her life she had a feeling she was bound for greatness, at first through her dream to be a famous actress and then tragedy 'prepared her'. It seems strange right? But compared to how the Dahlia and her ex-boy toy took their deaths happily knowing they'd be famous it starts to make sense... maybe Constance means because her son is the anti-christ which she knows. Even if it sets the world in flames ("destined for greatness") it will make her famous too.
Her behavior as an enabler for her husband, for Tate has finalized in her being an enabler for the ultimate form of evil...
At the end of the season The Harmons, Moira and the other good ghosts decide to scare people from living in the house to protect them from the evil ghosts. The house will eventually be sold to a property developer, and we don't know what will happen to them. At best, they're released. At worst, they're going to spend eternity on an empty plot of land or in an apartment building that will eventually be demolished when people refuse to stay there more than a couple of nights.