This question refers to Graff's conversation with Valentine in Chapter 9. Valentine does not really give a direct answer to Graff's question of why Ender keeps seeing Peter's face in the mirror.
Her first answer is that it is stupid. If ever there were two opposites, it would be Peter and Ender. Peter is a killer and Ender is kind. In saying this, she unknowingly gives Graff the answer he needs. Ender is seeing Peter's reflection because he feels like a killer. And the one person he associates with malicious killing is Peter. In his success at battle school, although Ender isn't actually killing anyone (physically), he is destroying them because they are weaker than he is, just as Peter was able to destroy things that were weak.
Ender does not see his success as a positive thing. He sees it as a reflection in himself of everything he always hated about his brother.