He was cruel to many students, but especially toward Neville Longbottom. It was no fault of Neville's but was instead because of Snape's love for Lily. Neville was almost the Chosen One, and the fact that he wasn't broke Snape's heart to look at him, which caused Snape to lash out cruelly at an innocent child.
We've already talked about how Snape hated Neville, and how he tortured him the most out of all of the characters. And even by Snape standards, this last action went too far. He once threatened to feed a potion Neville made to Trevor, his toad. If the potion was made properly, it would turn Trevor into a toadstool.
During his school days and shortly thereafter, he probably hated James the most. He seemed to be the ringleader bully, then married Lily.
Furthermore, she usually was the only one to raise her hand when the class was asked a question, and her being a Gryffindor compounded Snape's dislike of her. Not to mention her being a friend of Harry Potter, the boy whom he had to protect but also constantly reminded him of his father.
He is an exceptionally skilled wizard whose extremely cold and resentful exterior conceals deep emotions and anguish. A Professor at Hogwarts School of Witchcraft and Wizardry, Snape is hostile to Harry due to his resemblance to his father James Potter, who bullied Snape during their time together at Hogwarts.
For the Potter uninitiated, “always” is how Snape explains to Dumbledore in the final book why his Patronus takes the same shape as the one belonging to his long-lost love: Harry Potter's mother, Lily.
He was cruel to many students, but especially toward Neville Longbottom. It was no fault of Neville's but was instead because of Snape's love for Lily. Neville was almost the Chosen One, and the fact that he wasn't broke Snape's heart to look at him, which caused Snape to lash out cruelly at an innocent child.
So Snape was cruel, biased towards his house, and downright rude to his students: this we can't deny. Calling poor Hermione an 'insufferable know-it-all', for example, was pretty terrible.
He doesn't trust Snape as Dumbledore does, but he believes that he can detect any disloyalty. Moreover, Voldemort regards Snape as highly intelligent and therefore ' as a true Slytherin ' a person whose top priority is to take care of himself. To his twisted mind, selfless equals stupid.
Harry also happened to be Snape's least favorite student.
Snape Wouldn't Hate Her So Much
Harry reminded Snape of James, but if Harry had been a girl, he may have reminded Snape of Lily instead. Meaning Snape would have been a lot more eager to protect and help girl Harry throughout the series.
One of the reoccurring suspected villains was Severus Snape, Harry's potions professor. Snape was always bitter about something, and readers wouldn't find out until the end of the novels how much Snape was actually working to help keep Harry alive.
In the film, Snape is forced to fight Professor McGonagall after Harry Potter reveals that he was the one that killed Professor Dumbledore (at Dumbledore's bidding, though at the time the fact was unknown).
After Snape threatens to severely punish every student who fails to tell him about Harry's whereabouts, Harry reveals himself and members of the Order of the Phoenix back him up. Minerva McGonagall, however, steps up and singlehandedly fends Snape off using a fiery spell.
Because Minerva was his friend and they were fighting on the same side (even though she didn't know). So, even when Minerva was duelling to kill, he wasn't, and if he had kept on fighting he might have been forced to hurt her. He deflected every attack quite easily, even when Flitwick joined the fight.
Bellatrix Lestrange (née Black) is the first female Death Eater introduced in the books. Aunt of Draco Malfoy and Nymphadora Tonks. She was introduced in Harry Potter and the Order of the Phoenix. She is the most faithful member of Voldemort's inner circle.
Similarly to Harry, Snape's patronus represents a person. In the memories he gives Harry before his death, Snape shows Dumbledore his patronus, using the form to prove his love of Lily Potter, Harry's mother.
In order to preserve his position as a trusted Death Eater, Severus Snape was forced to tell Voldemort the true date at which the Order planned to move Harry.
“You disgust me,” said Dumbledore, and Harry had never heard so much contempt in his voice. Snape seemed to shrink a little, “You do not care, then, about the deaths of her husband and child? They can die, as long as you have what you want?” Snape said nothing, but merely looked up at Dumbledore.
Pursuing, McGonagall screams, "Coward!" Uncloaked, Harry and Luna rush inside, startling the other professors. Snape has leaped out the window. Harry thinks he must be dead, but McGonagall bitterly comments that, unlike Dumbledore, Snape had a wand and has learned a few tricks from "You-Know-Who".
Snape apparently told Draco to conjure the snake which I find rather odd. This is not exactly a good dueling spell.
Although Snape may not have projected his memory of James onto Harry, he still would have been predisposed to dislike him by virtue of him being James Potter's son. Harry being in Slytherin would not have changed this fact.
However, the prophecy went on to say that Voldemort would mark the child as his equal, choosing the boy that he believed would be the most dangerous to him. Dumbledore believes Voldemort chose Harry because Harry was a half-blood, just like Voldemort.
Bellatrix Lestrange was in constant competition with Severus Snape for Voldemort's attention. She was jealous of his magical potential and never trusted him. She was always on the lookout for something that would prove Snape's disloyalty to Voldemort.