Severus Snape went through difficulties since his childhood – not only in Hogwarts, where he got bullied and a broken heart but also at home, where he watched fights between his Muggle father and his Wizard mother. This bitterness, however, does not justify the harassment he did against his students. He did not accept his pupil’s insecurities and imposed fear in his classes.
Neville was one of his main victims. Snape always discouraged him by calling the boy an idiot in front of his colleagues. Once, the professor threatened to poison Neville’s pet toad if the boy made any mistakes in the potion he was brewing. It was such cruelty that, even though he lost his parents to sequels of torture sessions, Neville’s biggest fear was the professor, as his Boggart revealed in The Prisoner of Azkaban.
The boy was not the only one to suffer with the Potions Master. Hermione, since her first class was looked down on by the professor. When Draco attacked her and made her teeth grow over her mouth to her neckline, Snape made fun of her saying he had not seen any difference. This attitude impacted her so much she decided to alter her teeth permanently with magic.
The professor did not limit himself to insult the students only with bad comments. With Harry, he used physical violence. During Occlumency classes, when Potter found out about Snape’s past with his dad, the professor was so blind with anger that he threw the boy against his bottle cupboard.
Not only he harassed certain students, Snape constantly favored Slytherin. He awarded them with more points, cheated in the Quidditch championship and turned a blind eye on his house’s pupils, especially on Draco Malfoy.
His cruelty was not limited to students. In Harry’s third year, when Lupin had to miss work and Snape replaced him, he disrespected the teaching’s plan and even anticipated a class about werewolves so students had a distorted and prejudicial vision on Lycanthropy. Thirsty for revenge, Snape did everything he could to expose Lupin and make him get fired, losing the only job he had gotten in life due to the prejudice against him being a werewolf.
Even though he had all of these unethical acts, Snape was not fired from Hogwarts because he was Dumbledore’s spy and a key piece in the fight against Voldemort. However, when reflecting on the character as a professor, there is no other conclusion than that he had no condition to perform such occupation.