Thomas avoids the others for the rest of the day, totally preoccupied with his terrifying plan.
When night falls he finds himself in the Homestead with the same people he'd shared a room with two nights ago, plus Teresa.
When the Grievers attack they grab a boy named Dave, but Thomas runs out after them.
In a purposefully botched rescue attempt, Thomas attacks the Griever that has Dave and promptly gets attacked by three Grievers at once. He feels himself get stung several times and manages to somehow roll away from the attack and start running towards the Homestead.
People run out to grab him and carry him back to the Homestead.
He gets injected with the Grief Serum, and before he loses consciousness he whispers that he did it on purpose.