Ender learns that his final exam was really the third invasion. Why did Graff and the others deceive Ender?

After Ender has won the final battle, assuring humanity's safety from the Buggers, Ender is the only one who does not celebrate. He feels only sadness and anger. He is sad because he destroyed the buggers, and he did not want to hurt anyone. He is angry because he was manipulated perfectly, and Graff and Rackham got everything out of him that they wanted. Ender is really the only character who feels for the buggers, for he is the only one whose compassion extends not just to all human beings, but to all sentient beings. The buggers are intelligent life, and to kill them all is a horrible thing to Ender, even if there was no choice. Ender is angry because he still feels he should have had a choice. He knows, however, that the adults did what they had to do to save their species, and that whatever price he has to pay would be worth it to them. Graff and Rackham know that Ender feels betrayed, and the only justification they can offer is that they had no choice. Their manipulation was wrong, but it was the right thing to do for the survival of humanity. Ender Wiggin is a genius, and he understands people's motivations, but he is also tired of being used to fight other people's wars. Ender does not hate the buggers, and so this was not a war he would have consciously fought. At the same time, Graff, Rackham and the I.F. leaders knew that, and that is why they had to make it seem like a game.