It doesn't make sense; it doesn't have to, except for the last four words. River gets it -- "They're trying to make him angry" -- but she can't stop it working. "I'm sorry, sir. The Angels were very keen for you to know that." The Doctor answers their mean-spirited, ugly little unseating speech with one of his own: "Well then, the Angels have made their second mistake because I'm not going to let that pass. I'm sorry you're dead, Bob, but I swear to whatever is left of you, they will be sorrier... Yeah. I'm trapped. Speaking of traps, this trap has got a great big mistake in it. A great big, whopping mistake!"
He turns to the left, and the right, and they all offer him their trust. Their faith. "Always," grins his wife. Octavian offers the Doctor his gun, without hesitation. "I'm about to do something incredibly stupid and dangerous. When I do, jump." He hops, by way of example, but you can see it from here. "There's one thing you never put in a trap, if you're smart," he tells Bob. "If you value your continued existence, if you have any plans about seeing tomorrow, there is one thing you never, ever put in a trap." And what would that be?
The Doctor takes aims at the gravity globe, high above the cavern floor, just under the belly of the Byzantium, and shoots it apart with a soldier-priest's gun, but we already know the answer. If you're trapped in the Maze with the Devil, at least you know who the bad guy is. The bad guy is the one who trapped you there, the one who's so hungry He can't help turning you to stone.
That's how all stories start, and only a soldier-priest could know how much that really means, and so when the Doctor shoots the sky, they do it without asking: They jump.
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