Albus Percival Wulfric Brian Dumbledore was the greatest wizard the world has ever known. Dumbledore's prowess can be witnessed from the simple fact all the Harry Potter books were basically his plots and plans to defeat Lord Voldemort. He even planned his own murder.

Source: Pottermore

But let's be honest. We have all known Dumbledore's character wasn't as simple as black or white. The man was a genius but his choices in life and methods were flawed. And despite his role in defeating the Dark Lord, Dumbledore might just have been the worst Headmaster any school had ever seen. We have a few points to support the argument and here they are:

1. Dumbledore just dropped off Harry at his aunt and uncle's place without even meeting them.

The greatest dark wizard the world has seen has just fallen after trying to kill this boy. His parents are dead. He is an active target. Understandably keeping him with his only blood relatives is the best way to protect him from harm but his relatives are "the worst kind of muggles" who detest magic and would treat him like a slave from the 1800s. So instead of at least trying to make them understand, Dumbledore drops a 6-month-old baby at their doorsteps in the middle of a cold night without anything but a letter.

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2. He hired dangerous and incompetent people to the faculty, like Gilderoy Lockhart.

Gilderoy Lockhart was just ridiculous. Even the blind could see he was faking it. In his defence, he had a storehouse of books speaking volumes about his audacious works but come on! Dumbledore almost definitely knew Lockhart was a fraud. And then we have Lupin. Agreed that Remus Lupin was the best Defence Against Dark Arts teacher Hogwarts had seen in ages but think about how sad that is when you realise that he was a werewolf. Nobody is denying that he was a good man, who was martyred in the final battle. Dumbledore's intentions were noble. He wanted to end the bigotry against werewolves. But the stakes were too high. Even Hagrid, despite his love for animals and extensive knowledge, was a poor teacher who had a habit of raising dangerous creatures.

Source: Pottermore

3. He always knew who opened the Chamber of Secrets, but never told anyone.

When the Chamber of Secrets was opened for the first time, Dumbledore was a mere professor at the school. So he did not have much of a say in the matters of the school. After realising Hagrid had been framed, he let him stay on the school grounds. And it would be really naive to think that Dumbledore did not know who did it. The chamber opened when Lord Voldemort was still in school. He shared the same values as Salazar Slytherin and could speak parseltongue. And yet, when history repeated itself, Dumbledore did not inform anyone about Voldemort being the heir, leading to children almost losing their lives.

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4. He knew Harry was the 'chosen one' but didn't tell him anything and raised him like a pig for slaughter.

Harry probably thought of Dumbledore as grandfather figure who was guiding him to defeat the Dark Lord. Little, did he know that he, too, was a pawn in a much larger game played by Dumbledore. Dumbledore raised him on a path where he knew Harry won't care about his own life and in doing so, he groomed Potter to sacrifice himself when the time was right. And despite knowing that Harry was to defeat the Dark Lord, he never really gave him the resources needed to do the job. After his pre-planned murder, Dumbledore sets the golden trio on a mission to find Horcruxes with such vague clues, that it took even Hermione months to make sense of them.

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5. He was more invested in defeating Voldemort than actually running the school and looking after the children entrusted to his care.

One cannot deny that the second half of the second Wizarding War was just a game Dumbledore played to defeat Voldemort. Voldemort is dead because Dumbledore planned it that way. He was the General. But he was also the headmaster of one of the few wizarding schools in the world. He had a responsibility towards providing his students with a safe environment and a secure future, which he evidently did not pay much attention to.

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6. He was basically a manipulator who used everyone to achieve his goals.

Dumbledore manipulated Severus Snape into doing his bidding for the rest of his life, no matter how miserable he was. Snape played a double agent at great personal risk. Dumbledore was a mastermind when it came to getting the job done and often used questionable means to do so. He manipulated Harry Potter all his life. He manipulated Slughorn to get back to his older job. That, however, was necessary to defeat Voldemort. But then again, defeating Voldemort was one of the constant excuses Dumbledore used to justify his actions.

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7. He never introduced Patronus lessons in Hogwarts, despite having Dementors roaming around freely throughout Harry's third year.

The Patronus charm is difficult to understand and even tougher to perform. That being said, it is the only known way to fend against Dementors. During his third year welcome speech, Dumbledore warns students to stay out of their way but never introduces Patronus lessons to the students, who remained vulnerable to dementor attacks.

Source: harrypotterwikia

8. The school was just too unsafe for children. Who sends 11-year-olds into a dangerous forest for detention?

There were detentions in the Forbidden Forest. Who does that? Make them clean the store room or something. When you have clearly stated that the Forbidden Forest was out of bounds for first years, then why send four 11-year-olds, to investigate the murder of unicorns? And then there was a giant 3-headed dog on the third floor of a castle full of teenagers. Harry, Ron and Hermione (also Neville in the books) just happened to stumble across it. Imagine, other 1st year students, who are still getting used to the moving stairs stumbling across the beast or teenagers just visiting the place on a stupid dare! And then there's the Whomping Willow that had been planted to make sure that Remus Lupin would have a place to go on the full moon nights. But he had been gone for years and yet the murderous tree remained on the school grounds.

Source: Harrypotterwikia

9. Dumbledore is heavily biased in favour of Gryffindor.

Let's just give 200 some points to Gryffindor to make them the winners of the House Cup. Seriously, there are thousands of students who have worked tirelessly throughout the year to earn that prize. But you go on, give it to Gryffindor because you like Harry.

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10. He was willing to sacrifice everyone in pursuit of the "greater good".

Albus Dumbledore was power hungry, to begin with. Those who disagree can go read his history with Grindelwald and the death of his sister Ariana Dumbledore. Although he changed with time, his hunger for power evolved into his quest to do what he believed was for the greater good and like always, he was prepared to sacrifice his own life for the same. The problem, however, was that he was also willing to sacrifice others, even plotting the death of a 17-year-old boy and grooming him for it.

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He was a great man for sure, but he was a great man with a great many flaws.