Ender, after surrounding himself with a loyal army to keep from ever being in this situation, is cornered in the shower by his rival and some thugs while he is alone, tired, and naked. So he convinces Bonzo to fight him one-on-one and kills him. Accidentally.
Ender when he takes on Stilson at the age of seven and basically fulfils the fantasy of every small kid who's ever been picked on at school. The milage variance comes from the fact we only find out later that Ender killed the kid by doing so. As the teachers note — "Ender's not a monster. He just wins. Thoroughly."
