While there is a possibility that Ben is not the one who started the fire, the show strongly indicates that it was him. After he struggles to light a fire in his new secret cave, we see him steal matches and rope from outside the cabin. The final scene reveals that the supplies were not just for himself: he uses them to burn the Yellowjackets’ cabin down and block the doors.
Witnessing Nat being crowned the next queen of their cult and genuinely enjoying the reception, almost immediately after the horror of seeing the Javi’s butchered body, destroys Coach Ben. He believed Nat was the last vestige of humanity in the deadly wilderness. Now, Ben has no allies left and a target on his back.
The attempted murder of his students is disturbing, but it’s coming from a complex emotional place. Ben is physically and psychologically worn down from hunger, depression, and anger at the girls’ casual acceptance of cannibalism. Yes, they are also starving, but all he knows is that Javi has apparently been murdered by them for food.
One could argue that Ben’s act of violence makes him just as bad as the girls. He is also willing to kill for his own self-preservation. On the other hand, he has genuine reasons to fear the teenagers. They have turned into feral predators who enjoy the thrill of the hunt, beyond the simple need to eat. Perhaps Ben believes that it would be better for them to die in a single blaze rather than destroy each other one at a time or survive with the traumatic memories of consuming their friends. Or perhaps his motives are dominated by the realization that it’s kill or be killed in the woods, and Ben doesn’t want to be a victim.