Out of the four types of critters that were hurt by Ellie's classmates (snails by Rex, spiders by Victoria, butterflies by Phil, and ladybugs by Jay), only the snails and spiders play an active part in helping the Sprites kill their abusers during the night. Most butterflies and ladybugs are generally inactive at night, and neither are as good as the other two for creative means of killing their abusers (the snails have hard shells and slime to choke Rex, while the Spiders can web Victoria up until she suffocates).
The Sprites appear to drive an Anvilicious Aesop about protecting the environment, killing off Phil, Victoria, Rex, and Jay who harmed nature. But they take it way too far when they try to kill off Ellie for simply trying to save her friends (not to mention the fact that they kill unarmed and cornered teenagers with pleasure, like inveterate sadists), and as such, she becomes the one to kill them with salted foods when they corner her and refuse to back down. Calling it double Karma for the obnoxious classmates (harming the environment) and the murderous sprites (trying to kill an innocent just for freeing her classmates).
It seems odd that the Elder Sprite (who is one with nature and likely knows all the animals' genders) appears to misgender the bird as "Brother Bird" despite Ellie recognizing it as a female wagtail, and the bird being shown to have a nest with eggs inside Ellie's discarded waist pouch during the credits. However, as the bird had recently helped Ellie escape by giving the Elder Sprite a cheap shot in the face, it's possible the Elder Sprite likened her to Brother Rabbit, a trickster who always gets away from trouble by using his wits.
Ellie being the only survivor makes more sense in a mundane way because she was the only one who immediately ran away. Rex stayed and fought the Sprites, Jay tried to hide, and Victoria ran around in a panic.
After the death of her classmates and managing to escape from the sprites, there are chances that Ellie’s likely become traumatized from that experience.
From the bird trapped in a six-pack ring and the bloodied shoe, it's clear that Ellie and her classmates are not the first trespassers who entered the Sprites’s home and got killed by them (excluding Ellie who survived thanks to her love for nature, resourcefulness and an irresistible determination to survive). Just how many people have the sprites killed in the past!?
It's still at least somewhat early in the night when the fight begins, as seen by the moon being high in the sky. By the end, the sun is coming up. That few minutes we saw may have been several hours of the teens fighting for survival, sped up for our sake.
