Alby is the first one to accuse Thomas of being a traitor and a Creator spy. He thinks this could all just be a plan to lure the rest of the boys into the Griever's lair for a feast.
In a rage, he finally shares what he saw during the Changing: their lives before the Glade were horrific and none of them can go back. Alby then admits that he was the one who tried to burn the Maps and that he faked his own attack because he doesn't want to go back to the real world.
He remembers something called the Flare—a terrible disease, burned land, and that they'll all die if they go back.
When Alby storms out in a defeated rage, the other Keepers agree to try Thomas's plan, but only because if they're gonna die they'd rather die fighting.
Thomas reveals the final part of the plan: the Grievers can only kill one boy at a time, so when they storm the Griever Hole one person should sacrifice themselves to save the others… and then he volunteers.