What the series doesn't dwell on is her motive.
The ending is certainly effective when it comes to shock value, but how effective it is in revealing why Amma did what she did is arguable. After Mae's mother comes looking for her, Camille stumbles upon a tooth in the dollhouse. When she looks more closely, she sees the rows of teeth lined up. Then Amma shows up. Gillian Flynn's novel lets the reader sit with the revelations a little bit longer than the show does, and reveals why she went after these girls in particular: Jealousy. She targeted Natalie and Ann after Adora grew close to them; Mae, named Lily in the text, because she sensed affection from Camille. Camille realizes: "Amma controlled Adora by letting Adora sicken her. In return, she demanded uncontested love and loyalty. No other little girls allowed. For the same reasons she murdered Lily Burke. Because, Amma suspected, I liked her better." (Another upsetting detail Flynn adds: Amma uses Lily's hair as a rug for Camille's room.)
On screen, there's a hint of Amma's anger during the dinner scene with Camille's boss and his wife. Mae declares she wants to go into politics or journalism. Amma tells her she's just saying that to impress Camille, adding that she's a "kiss ass." It initially reads as just another bitchy comment. Soon, you understand it to be so much more.
Vallée works in a way that is more impressionistic than your typical murder mystery, but the truth is you could have seen that coming. It certainly helps explain Amma's creepy jaunt to the pig farm in the third episode -- the same site where Ann's bicycle was dredged up from a swamp. Or her panicked run into the shed in the woods when the fight breaks out at Calhoun Day. Amma's dialogue is peppered with lines that, in another light, sound like confessions. Meanwhile, those ghostly visions that Camille had of her little sibling -- like at the end of "Ripe" where Amma, roller skating, appears in her headlights -- seem prophetic.
But the biggest clue was out in the open the whole time: the dollhouse. As Camille peers in, there's a figurine propped up in the window, positioned like Natalie Keene's body was when found. That was Amma's calling card. Her fixation with the dollhouse was always unnerving. For a girl who often presented as older than her age, it was a habit that infantilized her. In the premiere, Amma, upon officially meeting Camille, takes her newfound sister to the toy, explaining that it's an exact recreation of the Wind Gap mansion in which she lives. Now we know how far Amma went to get her replica accurate: She used human teeth to mimic the ivory floors in her mother's bedroom. (Kudos to Reddit user "thunderheart26" for spotting this.)