This is Ender Wiggin after the end of the final Bugger war. He is an irresistible force. His unprecedented success in battle school and in the war are based on two remarkable attributes: Ender Wiggin never fails to understand his enemies. Upon understanding, he never fails to defeat them… Except for the one time that he did."In all the world, the name of Ender is one to conjure with. The child-god, the miracle worker, with life and death in his hands. Every petty tyrant-to-be would like to have the boy, to set him in front of an army and watch the world either flock to join or cower in fear."
Ender failed when he faced Stilson. It’s the only time that he didn’t understand his enemy, and it says as much in the text:
The Stilson and Bonzo fights are treated as essentially the same with regards to Ender’s moral responsibility, but they’re entirely different. Ender had every reason to believe that Bonzo wanted to kill him, and he understood Bonzo’s honour well enough to prevent it from happening by coaxing him into a one on one fight. It’s hard to make an argument that he had any understanding of Stilson’s frame of mind when he defended himself so brutally. We know his rationale; to hurt Stilson badly enough that he and his friends would be too scared to retaliate. But what retaliation was Ender expecting? Stilson wasn’t even taking the confrontation seriously. He was no Bonzo. He wasn’t trained and hardened in Battle School, and in all likelihood he wasn’t any more obsessed with saving face than any normal six year old.…Ender kicked out high and hard, catching Stilson square in the breastbone. He dropped. It took Ender by surprise he hadn't thought to put Stilson on the ground with one kick. It didn't occur to him that Stilson didn't take a fight like this seriously, that he wasn't prepared for a truly desperate blow.
Ender’s failure was allowing his own fear to keep him from understanding his enemy. Stilson was just a school yard bully who was neither strong enough nor malicious enough to cause the kind of harm that Bonzo intended, thus justifying Ender’s lethal response against him. For the likes of Stilson, a simple bloody nose and split lip would likely have been enough to deter further aggression. Even if it wasn’t, it’s doubtful that Stilson would have ever inflicted serious harm.
It’s ironic that the final test which Ender “passed†to earn his admission into Battle School was the only time when he utterly misjudged his enemy.