1) Esther asks the king to reverse Haman's decree calling for the slaughter of all Jews, as she cannot bear to see the destruction of her people.
2) The king cannot reverse an existing law, but allows Esther and Mordecai to write a new decree in the king's name protecting the Jews.
3) Under the new decree, the Jews are allowed to defend themselves against any attacks on the appointed day when Haman's original decree was to be carried out. This counters the original law while not directly reversing it.