The period before Rosh HaShana and Yom Kippur is always one of introspection, teshuva (repentance), and hopefully self-improvement. I wrote a series of articles a few years ago on examples of repentance in Harry Potter, and in the first article I listed Dumbeldore as an example of someone who clearly repented and changed his ways in the course of the story.
Deathly Hallows does a great job of portraying people's reactions to Dumbeldore's having made mistakes in his past. His opponents were thrilled to find something to "take him down" for, and even his followers (like Harry) were confused at the conflict between their image of their leader and the pictures they were seeing of his past. This is not different from all the revelations we see in newspapers nowadays, of politicians with questionable actions in their past.
The question is: Is Dumbeldore tainted by having consorted with Grindewald and planned anti-muggle activities? Or should he be judged only by who he is today?
This is a question that comes up often nowadays in how leaders are portrayed, both politicians and religious Jewish leaders. Is a President a worse President, or a worse human being, because of whether he inhaled drugs as a college student? Is a Rabbinical Scholar less wise, less worthy of following, or less leadership-worthy, because of whether he did something 30 years earlier that he wouldn't do today? Are the characters in the Torah less Sacred and less awe-inspiring if they did things that make them human and that we can learn from?
In the Harry Potter stories, I think the resounding answer is that Dumbeldore wouldn't have been Dumbeldore if he hadn't done what he did as a teen. Yes, it was painful for him later, but so much of his character, of his empathy, and of his understanding came from his living through his father's actions, his realizations about Grindewald, and his decision to move his life in the other direction.
This exact point is discussed in Harry Potter and Torah in a chapter titled Mudbloods Moabites and Moshiach. The chapter makes the point that the ancestors of King David, and thereby the ancestors of all Jewish Kings and of the Moshiach, have their roots in some very shady deeds. From Lot and his daughters to Yehuda and Tamar, we see why the Midrash says "I found King David my servant - where was he found? In Sodom!"
This is explained very clearly in the Hebrew book MiMa'amakim, which writes (page 95) as follows: In the period before Moshiach, the Jewish people will be in a dark and terrible state, and it will be the Moshiach's job to raise them to the highest levels. For him to have the power to do this, Divine Providence will have it that even the Moshiach's birth will reflect transforming definitive evil to the highest spirituality. Everyone alive will know that he has the ability to similarly transform all.
After all, don't we want leaders that have themselves accomplished the self-improvement and growth that we all aspire to ourselves? Don't we want leaders that can relate to us and inspire us?
The Talmud says that if we re-make ourselves on Rosh HaShana, G-d will credit us on Yom Kippur as a brand new creation, free of any of the baggage that we may have picked up along the way. Not only do leaders need to know how to do this, but we do as well.
I want to wish everyone a sweet new year and meaningful Rosh HaShana holiday. May we all be inscribed in the Book of Life and of all the Blessings possible.