Natalie and Ben
In previous seasons, Natalie and Ben had been an alliance in survival, working together on maps of the land, and Natalie encouraged Ben to leave the group because he does not belong with them anymore, given how far they are willing to go, as Ben refuses to engage in their rituals. In season three, Natalie and Ben's friendship is not only the thing that keeps Ben alive for an extended period of time, but it is also the thing that kills him. While it is uncertain if Natalie knows exactly where Ben is hiding, she knows that he is alive and out there somewhere, something she strives to keep from everyone else at the beginning of the season, knowing what they would do to him. At this point, Ben is the one everyone is blaming for the cabin being burned down, and once Ben is convicted, that is good enough for no one to ever question it or bring it up again.
Ben's trial, for as hard as Misty fought and may have been convincing, Shauna's rage was more than enough to encourage the jury to vote for Ben's guilt and execution rather than his innocence. Someone has to be at fault for the cabin burning down, and given how Ben was no help to Shauna when she gave birth and had been living on his own for months, he may as well take the fall. Still, when Lottie declares that Ben is their bridge home and they decide to keep him alive, it ultimately becomes a tragic story of Ben begging Natalie to kill him so he no longer has to suffer. With only one leg and an injured foot, Ben can not even try to escape successfully, and after watching the group put on masks and force-feed Ben, Natalie has had enough.
Natalie's humanity shines through as she aims to give Ben a humane ending, having had enough of his suffering. Their friendship is a key moment as Ben goes as far as to thank Natalie for ending his life, trusting her to finally put an end to the torture he has endured from the rest of the group. Ben's ending and Natalie's emotional turmoil at being responsible also play a role in helping to understand the perspective and actions of Natalie as an adult. Not only is it devastating that Ben had to die, but his death also led to Natalie being responsible for preparing his body for the group's feast, which ultimately created an opportunity to go home when the frog scientists stumbled upon their group of chanting and singing in utter horror. Ben's ending showed how much humanity Natalie still had in her at that point in time, as well as taking some of her remaining innocence with him, as she has officially become responsible for the group's first real murder.