The ending of this story of Esther serves as a reminder that God will accomplish what He is going to accomplish no matter how people try to get in the way.  Haman obviously was trying to exterminate the Jews (an effort also made in the 20th century by Hitler), but the Jews were God's chosen people, and they were not to be destroyed.  It's easy sometimes to think that we have ultimate control over what goes on in our lives, but in the whole scheme of things God is really the One who is in control.  Certainly we have choices, and God allows us to play a part in His unfolding drama, but He has already written the ending and has no plans on changing it.