By the time the final two Harry Potter movies came around, the standard of teaching at Hogwarts was abysmally low. With Amycus and Alecto Carrow routinely using the Cruciatus curse on students as a punishment (and encouraging older students to do the same), and Severus Snape at the helm of the ship, the school was almost beyond saving.
But even before the sharp downward spiral that was the result of Voldemort's hold on the Ministry, the teaching quality was always questionable. Even the best teachers at Hogwarts had some qualities that weren't ideal for their profession and, at points, even endangered the students.
Ruberus Hagrid: Too Careless
Rubeus Hagrid was, without a doubt, one of the best teachers at Hogwarts. He was well-meaning towards the students and had the best of intentions, truly caring about them. But he was given the job of teaching Care of Magical Creatures, which is perhaps not an ideal profession for a man utterly obsessed with them. He was so blinded with his fascination that he often brought creatures to class that put the students in a lot of danger, causing burns and even causing Draco Malfoy to get (deservedly, admittedly) mauled by a hippogriff.
Minerva McGonagall: Too Strict
Minerva McGonagall, head of Gryffindor house, was another good egg in the magical teaching community. Like Hagrid, it was obvious she deeply cared about the students and advocated for their well-being. But she could often be far too strict, and the way she spoke to Neville often embarrassed him. She was clearly old-fashioned but could have done with being a little more caring, especially towards the start of the series.
Sybil Trelawney: No Critical Thinking Skills
Sybil Trelawney was by no means a bad person, but her utter faith in Divination blinded her. She believed everything she saw and often unnerved the students by predicting unfortunate events. She hounded Harry about his tragic death until Umbridge arrived and, just to spite her, she announced that he'd live a long and happy life.
Horace Slughorn: Shameless Social Climber
Horace Slughorn was not an evil man — even if he was charmed into giving Tom Riddle the necessary knowledge to create Horcruxes — but he was incredibly selfish.
Even in his older years, he was a social climber, shamelessly favoring the students with high society Pure-blood connections and wealth. He would invite them to exclusive meetings and let them away with far more than he did others, which is highly unprofessional.
Gilderoy Lockhart: Wiped Others' Memories
Gilderoy Lockhart was a bad teacher from the moment he stepped foot on the castle grounds, utterly obsessed with himself and his own fame. As events unfolded, it was revealed that this wasn't the worst of him though — he had obliviated other wizards, taking credit for their stories and as a result, taking all their fame and money. So not only was he egotistical, he was a liar actively ruining the lives of others.
He got what was coming to him, though.
Albus Dumbledore: Highly Manipulative
At the start of the series, Dumbledore seemed to be a benevolent old man, a kindly grandfather figure.
Fans later found out he was actually power-hungry and manipulative. Although he was trying to do the right thing in his later years, it doesn't change the fact that he was raising Harry like a puppet and refusing to give him the full story about what was going to happen — even when it was apparent Harry was brave enough to do anything he could to help.
Severus Snape: A Jaded Bully
Severus Snape was, perhaps, the most divisive character in the whole series. Dubbed a hero by Harry himself at the end, he helped the war efforts against Voldemort hugely and took no credit.
But that doesn't change the fact that in his own classroom, he was an unnecessary bully. He ruined the confidence of many of his students, going as far as to insult the appearance of Hermione when she was only in her fourth year and making her cry. Why Dumbledore ever thought he'd be good for this job...
Dolores Umbridge: Dangerously Abusive
More abusive than Snape was Umbridge. She had many vile qualities, but easily her worst was that she physically tortured students as a form of detention by making them cut into their own hands. She also slapped Harry and would have performed the Cruciatus curse on him to torture information out of him, had Hermione not cleverly intervened.
Easily the most hateable character in the series!
"Mad-Eye Moody": A Death Eater In Disguise
Mad-Eye Moody never actually got to teach — he was Death Eater Bartemius Crouch Jr., parading as the ex-Auror. Barty Crouch Jr. essentially kidnapped Harry to be tortured and killed for Voldemort, utterly obsessed with his master. He did this while keeping Moody himself imprisoned in a trunk for the better part of a year.
Qurinus Quirrell: Literally Voldemort
The first Defense Against the Dark Arts teacher was Professor Quirrell, apparently a very nervous man. It turned out to be all an act when Harry discovered he had Lord Voldemort quite literally sticking out of the back of his head to survive.
Having Voldemort attached to one's self is undeniably a pretty terrible quality that's hard to beat. All that evil...