The Heroism of Esther
From a legalistic Jewish perspective we can understand Mordechai's instructions: Jewish law recognizes that a rape victim is not expected to give up her life, as she is considered a passive victim who unwillingly transgresses one of the three cardinal laws. On the other hand, any desecration, even of a more minor offense, if publicly known, becomes a "desecration of God's name" and therefore would also require martyrdom. This would explain Mordechai's insistence that Esther conceal her heritage, not just from the king, but from the Jewish community as well.
But the argument in favor of martyrdom is compelling on a psychological level. The martyr is assured respect by co-religionists in this world and an exalted status in the next. This, coupled with an act of defiance against the enemy, seems like a combination too good to turn down. Yet Mordechai does not allow Esther to take this course of action.