Harry Potter’s classmate and fellow Gryffindor, Neville Longbottom feared Professor Severus Snape the most. The fear got to a point where his Boggart (a magical creature that shifts shape into whatever it thinks frightens a person the most), manifested as Snape. Neville revealed this piece of information in the thirdHarry Potterbook -The Prisoner of Azkaban- after Snape sneeringly told Professor Lupin not to entrust the boy with anything difficult in their first Defense Against the Dark Arts class together.
Neville went scarlet in the book, while in the movie adaptation when Lupin asked what frightened him the most, he timidly replied, “Professor Snape.” Lupin’s first DADA class was about Boggarts and the “Riddikulus” Boggart-repelling spell, and boy did Neville ace it. The day-to-day reality was somewhat different, as Snape relentlessly and needlessly mistreated Neville, leaving the youngHarry Potterwizard traumatized.

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In chapter 37 ofHarry Potter and the Order of the Phoenix, “The Lost Prophecy,” Dumbledore tells Harry why Voldemort tried to kill him. It was because of a prophecy that Sybill Trelawney made to him in a room above the bar at the Hog’s Head Inn, shortly before Harry’s birth. This prophecy referred to a boy with the power to conquer Voldemort, born at the end of July to parents who thrice defied the Dark Lord. The odd thing about Trelawney’s prophecy was that it may not have meant Harry Potter at all, as there was another wizard boy born at the end of July to parents who served in the Order of the Phoenix. Moreover, as “The Lost Prophecy” reveals, both sets of parents had narrowly escaped the Dark Lord thrice. While one of the wizard boys was Harry Potter, the other one was Frank and Alice Longbottom’s son, andthe bumbling and fearful, Neville. The truth eventually comes to the fore, and the answer to the question as to why Snape mistreated Neville lies in his “Always,” i.e., his love for Lily and his dark past as a Death Eater.
Harry And Neville Were Born At The End Of July
Before Snape joined the good side, he was a Death Eater, committed to the Dark side and a loyal servant to the Dark Lord, Voldemort. He was the one who eavesdropped onTrelawney’s prophecy that foretoldthe coming of a boy who would be able to defeat Voldemort, and it is through him that the Dark Lord came to know the prophecy had been made. Snape did not know the full contents of the prophecy, as he was caught by the owner and thrown out of Hog’s Head. Nevertheless, he wasted no time in relaying everything he heard to his master. Chapter 37 ofHarry Potter and the Order of the Phoenixreveals Voldemort set out to kill baby Harry, “believing he was fulfilling the terms of the prophecy.” Had Snape known the prophecy would put Lily (and her expected child) in danger, he wouldn’t have relayed anything to Lord Voldemort. And while Neville and Harry were both born at the end of July, it was Voldemort who chose Lily’s son.
As to why Voldemort marked Harry and not Neville, Dumbledore in chapter 37 of the fifthHarry Potterbook -The Order of the Phoenixexplains that the Dark Lord chose the boy who he thought was most likely to be a danger to him. He chose a half-blood, just as he was, and not Neville who was a pure-blood. The fifthHarry Potterbook explains thatVoldemort set out to kill Harry while he was a baby as his information about the prophecy was incomplete. Snape had only heard the beginning, foretelling the birth of a boy born at the end of July to parents who’d defied Lord Voldemort thrice. The Dark Lord did not know the rest of the prophecy, i.e. attacking this boy would entail the risk of transferring power to him, and in the process, marking him as an equal. Thus, due to Snape partially hearing and relaying the prophecy, Voldemort did not take a more practical course and waited to see who among Neville or Harry looked more dangerous.

The Greatest Regret Of Snape’s Life
The question of why Snape chose to mistreat Neville can be answered through his enduring love for the woman who brought happiness and warmth into his life - Lily. Their story is traced to their childhoods, before joining Hogwarts when Snape lived down Spinner’s End which was nearby Lily’s home. As explained inHarry Potter and the Deathly Hallows,Snape observed Lily’s magical abilities from a distance and in fact, told her she was special. While Lily’s older sister, Petunia grew jealous of the pair’s friendship, they continued to be friends. They’d talk about Hogwarts, Azkaban, and whatnot. Snape and Lily’s friendship died slowly as he took an interest in the Dark Arts and kept company with people who used Dark Magic. The relationship took a nosedive in their fifth year when he called her a “Mudblood” in a fit of anger as she tried to intervene and stop James Potter and his friends from bullying him. Though he apologized later, she found it inexcusable stating he was set in his ways andaiming to be a Death Eater. It is pertinent to mention that Snape’s feelings for Lily were unrequited, and he lived with them even after she fell in love with and married James Potter.
While Lily and James fought in the First Wizarding War, Snape worked for the opposite side. Snape had a change of heart when Voldemort chose to zero in on the Potters. He had specifically asked his master to spare Lily’s life, and later come to Dumbledore to plead with him to take Potters into hiding. While yes, he cared little to none about James and Harry, it is Lily whom he wanted to be spared. Thereafter,he switched sides, and while he could never forgive himself for doing what he did to Lily, Neville was a living reminder of his dastardly mistake. Thus, the answer is simple - Snape mistreated Neville as he was nearly the chosen one. Had Voldemort picked Neville instead of Harry, Lily’s life would have been spared. Snape relieved his dark past through Neville Longbottom and therefore resorted to mistreating and insulting the almost-chosen one ofHarry Potter.