Ender sees Peter's face in the mirror while playing the game. This comes after he has recently fought his way out of a bad situation with older boys. It also comes right after a conversation in which Dink explains his opinions about how children should not be soldiers and the IF is merely using them for its own agenda.
Ender's reaction is mixed. Part of him seems to know that he is still different from Peter, but he is uncomfortable with how often he must use violence to solve his problems. He realizes that his teachers like the killer part of his personal character. This makes him feel terrible. He feels like a killer. While he disagrees with a lot of what Dink says, in this moment Ender seems to realize that Dink is right when he says that the adults are using them. They are turning him into a weapon. The image in this scene is a symbol of Ender's constant struggle to sustain his compassion and avoid becoming a killer.
This is a recurring theme throughout the book, as Ender struggles to find balance between peace and violence. Throughout the book, Ender continues to feel guilt over times where he turns to violence as the only solution to his problems. He does not feel like he has any say over his own moral future, since the adults are manipulating him at every turn.