Ender's Game is full of interesting quotes that speak to the themes of the entire work, and indeed the entire series as a whole. One of the best comes early in the book, when Ender is asked why he attacked the bully so violently:
"Knocking him down won the first fight. I wanted to win all the next ones, too, right then, so they'd leave me alone... You took away the monitor," Ender said. "I had to take care of myself, didn't I?"
(Card, Ender's Game, Google Books)
Throughout the novel, Ender is constantly under attack, physically or mentally, by forces larger and stronger than himself. To win, or just to survive, Ender needs to be smarter than his opponents, and so must use unconventional tactics. This quote illustrates how, even at a young age, he understands the importance of striking first when conflict is unavoidable; Ender knew that even if he beat the first bully, there would be more, and so he took steps to make himself look violent and crazy, so bullies would be cautious in dealing with him. Later, he uses the same tactics to win Battle School battles, personal fights, and finally the Bugger War.