This is the title of Chapter 11.
Third in a line, bred to be realistic,
You were a "Third" because you were the third child. You were allowed to be born because the government liked your siblings but thought they were too extreme.
Somewhat compassionate, not too sadistic.
You were between the extremes of your sister's compassion and your brother's sadism.
Taught a new game, I learned how to rack 'em.
You became very good at the war games at the school. Rack 'em is a play on Mazer Rackham, who trained you (credit to Hellion).
In spite of their cheating, I improv'd and sacked 'em.
The other team cheated to try to make the games fairer.
Surprised how you find me? The game ended victorious!
You passed the final test.
Ironically infamous, disgraced and inglorious.
This resulted in the annihilation of the buggers' home planet.