As I read the book of Esther, I see God's sovereignty. He allowed Haman to come to power but he also allowed Esther to become queen.
I noticed Mordecai's words:
Then Mordecai told them to reply to Esther, “Do not think to yourself that in the king's palace you will escape any more than all the other Jews. For if you keep silent at this time, relief and deliverance will rise for the Jews from another place, but you and your father's house will perish. And who knows whether you have not come to the kingdom for such a time as this?” (Esther 4:13-14)
This sounded like the same idea I read in Job:
But He is unchangeable; who can oppose Him?
He does what He desires.
He will certainly accomplish what He has decreed for me,
and He has many more things like these in mind. (Job 23:13-14 HCSB)
Mordecai urged Esther to step into God's plans for her life, to accept her calling and trust in God's protection.