Ender does begin to despise his superiors but with good reason; the men in charge, like Colonel Graff and Major Anderson, have completely manipulated him and violated his trust repeatedly. While it is certainly not okay just to hate the people in a a role of superior power based on that fact alone, Ender's feelings are really more of a product of Graff's continued manipulation. The letter from Valentine is the 'final straw' for Ender. From Ender's perspective, his sister represented a sacred trust, and for the International Fleet to manipulate her into writing that letter was a complete betrayal.
Ender's empathy and sense of understanding make him a dynamic and endearing character, and the other characters, particularly Graff, deceive and cheat Ender multiple times, playing on his sense of goodness and compassion. Ender does begin to dream of defeating his teachers at Battle School, but only out of a sense of complete betrayal.