Kill Creatures by Rory Power
A year after Nan's best friends Luce, Edie, and Jane disappeared, the town of Saltcedar is still mourning. So is Nan-- or so it seems to everyone else. Saltcedar holds a memorial for the girls at the river to commemorate the anniversary of their disappearance, and during it, a body of one of the girls is found.
A body that is alive, not dead.
Nan should be relieved that one of her besties is still alive. But she's not, because Luce should be dead, and Nan knows this because she is the one who killed them all.