We’ve heard the tragic sob story about your friendzoning and you playing a double agent to He-Who-Must-Not-Be-Named. We’ve seen your memories in the pensieve, and we’ve accepted that you really were on the Order of the Phoenix’s side after all. But Snape? You still suck. And here’s why.

1. In "Prisoner of Azkaban," Snape substitute teaches for one of Remus Lupin’s Defense Against the Dark Arts classes. Despite the instructions left by Lupin, and the help of Hermione to explain to Snape where the class left off, Snape dives headfirst into the werewolf section. This is problematic mainly due to the fact that his intentions were that a student would discover Lupin’s true identity. In doing so, he also revoked Lupin’s right to teach his class about werewolves and to view them as people with “furry little problems” rather than as monsters, as Snape chooses to present.
2. He may have secretly been on the right side, but he only came back from it when the love of his life was murdered. Snape was dutifully working for Voldemort right up until he found out that the prophecy he overheard meant that Lily would be killed. He never became a double agent or switched sides out of loyalty to justice or to Dumbledore—rather, he did it because he didn’t want one person to die. Who cares about the other hundreds of lives that were lost due to the terrorist organization that he was a part of? Whatever. Why should that be his problem? Then, he went on to imply that Dumbledore failed him and that Dumbledore should have saved Lily in return for Snape coming back to the good side.
3. He unnecessarily bullied students to the point where he was Neville’s boggart. He may have had an awful childhood, and ran with a bad crowd, but nothing gave him the right to take up a position of authority and use it to torment children. A boggart takes the shape of the one thing you fear the most, and Neville chose Snape. That’s despite the fact that there was a mass murderer who ran a terrorist organization around, a suspected mass murderer who just broke out of Azkaban, and a Death Eater who tortured his parents to insanity. Nope, he was afraid of Snape above all of those things. T for troll.
4. He didn’t care about Lily enough to save her husband and son; he only wanted her to survive, even if that doomed her to a life of misery. He literally could’ve asked Voldemort to spare the entire Potter family, but instead he requested that only his teenage infatuation be saved. Who cares if she doesn’t have her chump husband or her only son? Yeah, whatever.
5. He called his best friend the worst racial slur in the wizarding world, and then acted wounded when she would no longer speak to him. In the heat of the moment, he referred to Lily as a “mudblood.” He then tried to tell her that he was sorry and that her blood status didn’t matter, even though his best friends and the organization he was a part of believed exactly the opposite. It’s no wonder she left him; no friend does that.
6. In the "Deathly Hallows" film, he completely ignores James’ body and Harry crying in his crib to clutch Lily’s corpse. There’s really nothing else to add to this except that it’s creepy as heck.
7. I would sugar coat this, but there’s just no way. Snape’s hair is just so gross. It’s a curtain of greasy black slime that covers his eyes constantly. Maybe Lily would’ve stuck around a little longer if he had discovered shampoo, or basic hygiene. The world may never know.
8. His patronus was a doe, which is the same as Lily’s. James’ patronus, on the other hand, was a stag. While James and Lily complimented one another, Snape mimicked Lily. He was so infatuated with her that he didn’t notice he wasn’t a good match.
9. He broke into Harry’s mind repeatedly and grew furious when Harry defended himself. This is ludicrous for multiple reasons. First off, he gave Harry absolutely no warning what he was about to do and no way to defend himself except to “close his mind.” Instinctively, Harry used his wand and involuntarily saw Snape’s worst memory. Because he is the bigger person, obviously, Snape then refused to give Harry lessons ever again and threw him out of his office. Mature.
10. Snape was head of the Slytherin house, and showed favoritism towards his students (namely, Draco) to an abhorrent degree. While he would punish Gryffindors for even the slightest misstep, he would allow students of his house to get away with bullying, snark, sarcasm, and even outright degradation towards their classmates.
11. He threatened to expel a student who made a ruckus in his class. Granted, Hermione had been stealing ingredients from his private stores at the time, and Harry had been selected to create a diversion. A dung bomb in a cauldron seems to be a slight misdemeanor unworthy of expulsion, however.
12. He made fun of Hermione’s appearance and intelligence in front of the entire class on multiple occasions.

More or less, he’s just a scumbag who can’t teach and who is the worst possible excuse for a redemption arc that there is in this entire series. The one character I can think of who is less desirable is Umbridge. Severus Snape is the worst and there is really just no argument about it.