Hogwarts School of Witchcraft and Wizardry was home to many students and several professors. During Harry's education, there were multiple Defense Against the Dark Arts professors. One of which, Gilderoy Lockhart, entered the castle during Harry Potter and the Chamber of Secrets.
Although Severus Snape did not become a Defense Against the Dark Arts professor until Harry's sixth year, the Hogwarts students already had plenty of experience with him from Potions. His interest in the Dark Arts and history as a Death Eater made him dangerous. While Lockhart was not intimidating like Snape, his incompetence and arrogance made him unpredictable and dangerous.
Snape: Favored Slytherins
Many consider favoritism to be a negative trait in teachers. It gives an obvious bias toward some, leaving others to be ignored or penalized. Professor Snape made it clear that he favored Slytherin House above the rest. He would ignore Draco's taunts and blame the Gryffindors and taking points from them. Snape's actions made it harder for the students from other Houses to do well in Potions. It was abundantly clear that Snape favored Slytherins when he would call out people from other Houses for incorrect work but would praise Draco.
Lockhart: Fame
Gilderoy Lockhart thought a lot of himself. He was his own most enormous fan. Framed pictures of himself hung around his office, and Lockhart's quizzes were about his interests and adventures. However, if there was any rival for fame, it was Harry Potter.
With no interest in being famous, Harry tried desperately to stay out of the spotlight, but Lockhart would not have it. Lockhart tried to latch himself to Harry to increase his own popularity. He would boast about how Harry had arrived at school in a flying car was a publicity stunt. He would not listen to Harry, trying to contradict Lockhart's statements.
Snape: Held Onto A Childhood Grudge
Severus Snape had been in love with Lily Evans for nearly his entire life and loathed James Potter for almost as long. James and his friends often bullied Snape in school. However, perhaps the last nail in the coffin was when Lily fell in love with James. Snape was hurt because Lily did not reciprocate his feelings. He never got over her for the rest of his life. Snape carried his anger toward James for years; it is still raging within him when he meets Harry. Snape took various opportunities to insult James in front of Harry.
Lockhart: Stole Adventures From Others
Gilderoy Lockhart was not talented at much. But, his most significant accomplishment of all was memory charms. Lockhart had erased the memories of accomplished wizards and instead had claimed the successes himself. While many had quickly fallen under Lockhart's spell, others were not as easily persuaded. Lockhart's entire life had been a giant lie that he eventually reveals to when all his lies land him being asked to help with the Chamber's monster. Lockhart's entire reputation was built on a series of lies. Lockhart later gets a taste of his own medicine when a memory charm goes wrong, landing him with no memory of who he is.
Snape: Teaching Style
While the half-blood prince's book may have helped Harry in Potions, as Harry's teacher Snape was the opposite. Snape was not a helpful professor in his classroom. He was harsh and spent more time insulting the students than he did teaching them the techniques. Several students had a difficult time in class based on the way he taught, mostly Neville Longbottom.
Neville was so distraught by Snape that the Potions Master was his greatest fear in Harry Potter and the Prisoner of Azkaban. While Snape may have been knowledgeable in both Potions and Defense Against the Dark Arts, that does not excuse his treatment of the students.
Lockhart: Removed Harry's Bones
It became clear relatively fast that Gilderoy Lockhart had no idea what he was doing. From failed Defense Against the Dark Arts Classes to ordinary spells, Lockhart was collapsing all over the place. Harry was on the receiving end of Lockhart's disappointing healing spell. What was meant to mend Harry's arm only made the situation worse. Due to the spell, Harry ended up without bones in his arm altogether. Harry had to stay in the Hospital Wing, under the watchful eye of Madame Pomfrey, until his bones grew back again.
Snape: Directly Insulted Hermione And Neville
Not only was Snape a problematic teacher with criticism, but he also held it in a more personal regard. Hermione is caught in the crossfire between Harry and Draco's fight, leaving Hermione with growing front teeth. When Snape catches sight of Hermione, he ignores it, saying he sees no difference in her appearance. He also directly called Hermione an "insufferable know-it-all" to her face. Snape also had no problem threatening Neville's toad, Trevor. He could be exceptionally cruel to Neville, insulting his magical ability. Neville was so scared of Snape that the Potions Master turned out to be Neville's Boggart.
Lockhart: Incompetence
Since Lockhart had no idea how to be a teacher, he wasted a year of Defense Against the Dark Arts for everyone. His presence at the dueling club only further showed how unprepared he was to handle a legitimate duel. His attempt to use Pixies went spectacularly wrong, and he raced out of the room, leaving them with the remaining students.
Hermione, Harry, and Ron had to gather the Pixies, cleaning up Lockhart's mess for him. Lockhart also signed Hermione's request note for a book in the Restricted Section of the library without paying attention to what book she wanted.
Snape: Active Bullying
Hermione Granger and Neville Longbottom were not the only students that Snape targeted. One primary student that Snape loathed beyond anyone else was Harry Potter. No matter what Harry did, Snape found a way to blame him for trouble and wished to see him in detention. Snape even used Occlumency lessons to bully Harry Potter. He would insult his students regularly and imply that he would harm them. Snape wasn't just a bully to his students; he also was to colleagues. While Severus Snape and Remus Lupin did not have the best history together, they had grown up since their days as Hogwarts students. Yet, Snape lets everyone know that Remus is a werewolf, knowing that people would want Remus to be removed as a Hogwarts Professor.
Lockhart: Attacked Harry And Ron
Gilderoy Lockhart was a very problematic professor. Not because he was in league with Voldemort like others were, but because he was unpredictable. Unpredictability can be dangerous, and Lockhart proves that when he points his wand at Harry and Ron. The boys had gone to Lockhart for help saving Ginny and defeating Slytherin's Monster. After discovering Lockhart is a fake, they still bring him along anyway. However, Lockhart gains control of Ron's broken wand, aiming it at the two Gryffindors. However, rather than causing Harry and Ron to lose their memories, the spell backfires, causing Lockhart to forget everything, including his identity.
