Answer:
Step-by-step explanation:
Based on the given excerpt from "Ender's Game" by Orson Scott Card, it is not clear what word Graff meant to end his original thought with. The text only states that Graff "almost said something" but stopped himself before he said it.
As for why Graff had to be sure of Ender's motive, it is likely because Ender is being groomed by the military to become a strategic commander in a war against an alien race known as the Formics. In order to do this, they need to ensure that Ender is motivated to win the war and is not just going through the motions or playing a game. By understanding Ender's motivation, they can better prepare him for his role in the war effort.