Severus Snape is a prominently featured character in all seven Harry Potter books, written by J.K. Rowling, and is the subject of much debate. Was he just doing what he could in his difficult position? Are his actions and behavior truly inexcusable? Do his actions in the last book redeem him? Is he irredeemable? Based on what we know of Severus Snape, his upbringing and life, you can conclude that he shouldn't be treated with the reverence a good portion of Harry Potter fans give him, as he was an awful person who made questionable decisions and made himself truly detestable. Some, perhaps even yourself, immediately start to make excuses; He's a product of his upbringing! He was bullied as a child! He loved Lily! Fear not, friends, we'll address all of those points, I promise.
The first we'll address is the common defense: He was a product of his upbringing. While, yes, his upbringing was something akin to awful, with a drunken, perhaps even violent, magic-hating muggle for a father, and a witch for a mother. From what we can gather, they quarreled and fought more often than not, and often neglected the young Severus. He grew up in the dirty, run-down Spinner's End, and was often seen in rags that were huge and dirty. This unfortunate upbringing was most certainly influential in the formation of his later personality and mentality. But wait! I seem to recall another little boy placed in an incredibly similar position: neglected and uncared for by the family he lived with, muggles who couldn't stand the mere mention of magic and were without a doubt abusive, clothed in huge, used, worn through hand-me-downs and alienated from all other children he meets by his cousin and aunt and uncle. That's right, one Harry Potter! Last I checked, Harry Potter was able to overcome his upbringing, becoming a brave, kind person. At a certain point in a person's lifetime, they are, believe it or not, able to decide and work to behave differently than the way they are raised to, and while difficult, I am certain Snape could have found some time to revise his behavior in the 38 years he lived.
On another note, yes, he was bullied by the Marauders during Hogwarts. We see this in the memory Harry views in the Pensieve in Snape's office, and in nearly every interaction with either of the remaining Marauders, Remus Lupin and Sirius Black, and even in some of his remarks to Harry. It is very clear that he was the subject of bullying through his 6th year at Hogwarts at the hands of James Potter and Sirius Black, at the very least. James Potter was noted to hex innocent students for no other reason than he could, so there is no doubt he was a bully, and Sirius Black was raised with Pureblood ideals (don't start on me about hypocrisy, they were 11-16 at that point it is expected they act as products of their upbringing). James Potter however, loses some of the less desirable aspects of his personality, we're told, and even saves Snape's life during what we assume was their 5th/6th year, while Snape drifts further to the Death Eaters. Less mentioned is the fact that Snape also cursed/jinxed/hexed them at any available opportunity and actively tried to have them expelled. Snape clearly carries resentment for Lupin and Black into his adulthood. Lupin we can assume has tried to make some sort of amends and is mostly, if not always, civil in his interactions with Snape, though we can see the Black still despises Snape. So much so that he would, perhaps, let him sit in the worst place in the world when he knew he was innocent? Tell me, how would Snape, a Death Eater in the inner circle, not be privy to the information that Pettigrew was the one who betrayed the Potters? You may start to say, "Oh, but Snape made Remus Wolfsbane during Harry's 3rd year, so he's not that bad!" Well, yes, he did, while also actively trying to get Remus fired. Snape was not subtle nor did he try to conceal his hatred for werewolves, such as Lupin. He went so far as to teach the students about werewolves and write an essay on the best ways to identify and kill them, hoping that some students would be clever enough to put two and two together and realize Remus was a werewolf. On the night Remus transforms in front of Sirius, Ron, Snape, Harry and Hermione, are several examples of his resentment and poor overall character. Before Remus even transforms, Snape attacks his whilst he's attempting to explain the situation and then said, "I'll drag the werewolf. Perhaps the Dementors will have a Kiss for him too." Moving on, he continued to ignore anyone trying to explain the situation, notably Sirius, who was relatively compliant, only fighting back (without magic) once, after Snape attacked Remus. After they make their way back to the castle and Snape is delivering his hero story, he is enraged when he finds Sirius gone and he wouldn't be getting the Order of Merlin for his capture or see either Sirius or Remus Kissed or jailed, innocent as they were. The next day, he tells not only the whole school, but the entire magical world the Remus is a werewolf, forcing him to resign. Looking forward to Order of the Phoenix, we see the lasting consequences of this when Sirius mentions to Harry a law, written by Dolores Umbridge two years ago, against werewolves, that made it nearly impossible for Remus to get a job. What happened two years ago? The Prisoner of Azkaban. So out of childhood-fuelled spite, Snape ruined not only the life of Remus Lupin but of every other magical werewolf in Great Britain. There's very little I can say defending the relationship between Snape and Sirius, as the both clearly hate each other.
Going back to the subject of his being bullied through Hogwarts and his unfortunate childhood years, we'll now transition to his years as a teacher at Hogwarts. Snape abused his position of power as a teacher to be unfair to students (giving points to his own house and taking excessively from others), and bully the children he taught. That is a fact. (It is understood that Potions requires a certain level of discipline as it can be dangerous with the volatile ingredients and combinations, but you must compare behaviors for managing this between the only other Potions professor we see, Slughorn, and Snape, and we'll save that for another day). Regardless of his own schooling experience, he has no right to bully the impressionable 11 year olds you are meant to teach. Children, whether they show it or not, at that age look up to authority figures like teachers, and have to deal with this despicable behavior for all seven years they went to school. Some of these students have just been introduced to this world and Snape is the teacher they have? Snape despises these children, which doesn't really make sense when you know that he is a professor. We know that he wanted the Defense Against the Dark Arts job, but the fact he didn't get it does not mean he should take it out even remotely on his students. Harry was bullied from the moment he first interacts with Snape, who hates him because he looks like James. You can be sure that he would not have hated Harry half as much if he resembled his mother, perhaps having fiery red hair instead of James' black. In his first interaction with Snape, Snape asks him OWL level questions and is cruel and demeaning when he can't answer them. Many fans have read into this a deeper message, which there very well might have been, but consider it from Harry's perspective and a first year on his first day of class. There's no way an 11 year old would have picked up on that. As he was being asked these difficult questions and being singled out in front of his peers, he wasn't looking for a hidden message, he was probably just hoping to make it to the other side without being entirely humiliated by his malicious teacher. Snape may have protected Harry from some major threats but he made part of his life miserable for no apparent reason. Moving on, he bullied Neville Longbottom to such an extent that at 13 his worst fear was his Potions professor. This is a boy who had his parents tortured to insanity by well known Death Eaters. Think about that for a minute. On a separate occasion, he threatened Neville with the potential death of his pet, Trevor, should Neville fail to make his potion correctly, knowing full well that Neville struggled with the subject. Snape constantly refers to Hermione Granger as an insufferable know-it-all and demeans her. At another time, he insults and mocks her, then watches her run out in tears after she's jinxed, causing her teeth to grow drastically, and she later permanently changes the appearance of her teeth to avoid being mocked. I think you're starting to get it, so on to the next thing we go.
And last, but not least, my personal favorite, "But he loved Lily!" Ok, let's look at some things, then I'll tell you what I think. As a child, she was his only friend both before Hogwarts and for the first half(ish) of his schooling. Snape makes his disdain for muggles and muggle-borns well-known from the moment he's sorted into Slytherin, so why would Lily be the exception? From what we can gather from the books, Lily did excuse his increasingly bigoted behavior a few times, until he called her a wizarding racial slur, which he then tried to defend by saying she alone was different. When he was caught in a stressful situation where she defended him, he immediately turned and called her the wizarding equivalent of the n-word. He destroyed their friendship singlehandedly, and when she didn't immediately excuse and forgive him for this, he turned and increased his involvement in the wizarding KKK. Snape declaring his love for her changes nothing. When you look at it, his "love" for Lily was more of an obsession and kinda stalkerish. When he discovers Lily's body (after walking past her husband's corpse and ignoring her child to cradle her lifeless body), he also finds a photo and a letter, you know the one. He rips Lily out of a picture of her laughing with her husband and child and rips off her signature ("Love, Lily") to a close friend. On the subject of Lily's family, Snape did not care if James or Harry died, so long as he could have Lily. Did he really think that even if he "saved" her by asking Voldemort to spare her life, that she would just forget about her family and be with him? If Neville had turned out to be the child targeted by Voldemort because of the prophecy, Snape wouldn't have defected to become a double spy, he would still have been a Death Eater.
As many of us read the books, we disliked Snape more and more until the final book when that one sliver of information was revealed, causing many to suddenly view him as a kind of saint. Snape had no real excuse for his downright malicious behavior towards his students for years, his attitude as an adult towards other people and his overall actions and disposition.