There are two desires in man that lead to evil in the Harry Potter series. The first, obviously, is the fear of death, personified by you-know-who. But the other, perhaps less noted, is the abuse of power. In fact, power itself becomes a greater and greater concern as the series continues. This is exemplified in 5th book by the extremely evil Delores Umbridge, who will to do anything for power, including ordering Dementors to attack the young Harry, using the cutting curse to teach someone their place and using the torture curse as a means of interrogation. She is portrayed as sadistic, enjoying the fear she inspires, and the pain she inflicts, both in book 5 and in book 7. Like any true power seeking bureaucrat (similar to Percy Weasley), she seems willing to do anything as long as it increases her power. When the Ministry is Anti-you-know-who, she is too, and when the Ministry is Pro-you-know-who, she is too. And when she has that power, she is ruthless in pursuit of both her own and the ministry's goals. And like any true power seeking bureaucrat, she seeks to enlarge her authority when anyone trumps hers. It is a personal affront, humiliating, to have anyone overrule her. Step by step, she becomes in book 5 the ultimate authority at Hogwarts, getting the power she craves.
To a smaller degree, this concern with power plays itself out with Harry's relationship to the Ministry of Magic: The ministry wants to use Harry for its own purposes, and when Harry is reluctant, it uses the threat of force to make him comply. Harry's anger at this misuse of power is a minor part of the work, but it is a consistent theme. Harry's distaste is supposed to mirror our own at governments that play politics with important issues, that use a variety of threats to make people do their bidding. This culminates in the Ministry of Magic using those powers to institute anti-Muggle legislation, similar in scope and depth to the anti-Jewish laws in Nazi Germany, surely not by accident. The wizard world is to be Mugglefrei.
All this leads to Albus Dumbledore. Repeatedly in the books, Wizards are shocked that Dumbledore never became the Minster of Magic. Why this never happened is never explained, until the 7th book. The usual explanations people give are that he, in essence, wants to stay in his ivory tower, that he is too gifted a wizard to concern himself, or be interested in, the daily grind of governance. But it turns out that this is not the case.
In the 7th book, the dead Dumbledore comes back visit Harry after Harry 'dies.' And in that, we learn something of Dumbledore's past that was described earlier in the book: Dumbledore made friends with another powerful wizard, Grindelwald, who has plans to take over the muggle world for their own good. Dumbledore is gripped by this view, persuades himself that they could do great things. He revels and enjoys losing himself in this vision. He suppresses his intuitive grasp of his new friends more evil side, and plans the take over of the world. In doing so, Dumbledore forgets his real responsibility, to take care of his little sister, who as a child had been abused by some muggle boys, and never was the same again. When his brother confronts him, a fight breaks out, and Dumbledore's sister is dead.
This is meant to show us, as Dumbledore says, that Dumbledore, too, has a taste for power, that he too, is drawn to its misuse. He wanted to rule the world, he enjoyed thing about having that kind of power, and he loved it so much, he did not take care of his responsibilities. He says "I had proven, as a young man, that power was my weakness and my temptation. It is a curious thing, Harry, but perhaps those who are best suited to power are those who never sought it." Indeed, he says as well, "I was offered the post of Minster of Magic not once, but several times. I had learned that I was not to be trusted with power." For Rowling, the desire for power, ambition, is a source of great evil. The people to trust, therefore, are those who do not have this desire. It is Harry who does not have it, and it is Harry who we can trust.
Plato, long ago in his masterpiece The Republic, writes of similar issues. He wanted to show that it is always best to be just. A just ruler, Plato argues, does not rule for his own glory, or for his own interest. Money and honor are not what the truly just person seeks. "Good people won't be willing to rule for the sake of either money or honor.... They won't rule for the sake of honor, because they aren't ambitious honor-lovers."(Rep. 347b-c) Ambition, like with Unbridge and the young Dumbledore, can lead people to do unjust things. Instead, good people "approach ruling not as a good, nor as something to be enjoyed, but as something necessary." Indeed, in a city of good men, "the citizens would fight not to rule, just as they do now in order to rule."(Rep. 347d) A short while later, he writes "perhaps those who are best suited to power are those who never sought it." (Okay, that was not really Plato, but Rowlings. But Plato could have written the exact same thing. Well, in Ancient Greek. Translated of course.)
The Harry Potter books tell us to be suspicious of if not downright hostile towards people who want power. Plato argued for the same point in his Republic.