Ender spares the life of just one enemy Formic instead of genociding them all.
Saul spares the life of just one enemy Amalekite (Agag) instead of genociding them all as God Commanded. Haman, a descendant of that survivor, goes on to wage genocidal warfare against the Jews.
Tell a continuation of the Ender saga in which the Formics, descendants of the egg Ender spared, return many generations later and wage genocidal warfare against the humans. Perhaps with some science fiction time travel (Star Trek: First Contact), that second war is the one Mazer Rackham wins. Celebrate yearly the anniversary of Rackham's miraculous victory.