When Ponyboy and Two-Bit go see Dally in the hospital, he asks about Johnny. From the responses of Ponyboy and Two-Bit, Dally realizes that Johnny may die from his injuries. This realization makes Dally fearful, as his love and his protectiveness for Johnny are strong. For Dally, fear transforms immediately into rage, and he needs a weapon to exact the revenge he suddenly desires in an intense way.
Though Johnny's decision to try to save the children from the fire had nothing to do with the Socs, Dally blames the Socs and hates them for having privileges that he and Johnny lack. That night, the rumble between the greasers and the Socs to end all rumbles is scheduled, and Dally wants to borrow Two-Bit's prized possession to make sure he is prepared for the rumble. Dally, in his anger and grief, is ready to hurt someone at the rumble with the Socs later in the novel, and Two-Bit hands over the weapon without a second thought. After all, according to Ponyboy, "Dally knew how much that knife meant to Two-Bit, and if he needed a blade bad enough to ask for it, well, he needed a blade."