
It may look as if his alliance betrayed him. But does his alliance have a choice? In this game, the only person who's been really hurt by the decision is John. Because he lost his chance at winning.
He know he's no longer valuable. His muscles have grown stiff. He has a deficiency. Which can only be corrected if he gets to rest and gets back to eating three balanced meals a day.
Besides, if that happened to anyone of his alliances, he will most likely vote to keep them out too.
Even if they kept John, he is not going to last the individual challenges now that the merge is about to happen. Can you imagine John during an endurance challenge? He will be the first one shivering from the cold out there.
His alliance didn't betray him.
His best friend in the game (Jace) did not betray him.
His body did.