One Piece Chapter 4, page 17: Chapter 4: The Great Captain Morgan

Luffy grabs Helmeppo, saying he's been looking for him, and drags him inside the fortress while Helmeppo screams for his father's help. Morgan orders his men to kill all the rebels. Down in the parade ground, Coby runs up to Zoro to untie him. Zoro warns Coby that he's being reckless and will get killed, but Coby bravely declares that he can't stand to see the Navy act improperly and wants to become a proper Marine, just like Luffy is going to be the King of the Pirates.