Henry applauds the feats of his men, and encourages them to greater heroics. Exeter informs the King that the Duke of York has died a heroic death on the battlefield, along with his friend the Earl of Suffolk.
Henry then is made aware of the French counter-attack, and orders his own soldiers to kill their prisoners.
Henry's decision to order his men to kill their prisoners is a difficult one for a modern audience to stomach. Shakespeare is merely following his source, the English historian Holinshed, who reports this incident. Presumably the order is given because, with the French about to launch a counter-attack, there is a scarcity of men on the English side, so they cannot afford to delegate some men to guard the prisoners.