I have been thinking about Severus Snape a lot lately - ever since I'd come home from a Lauren Fairweather concert where she performed songs from her brilliant Prince's Tale album and reduced me (and the rest of the audience) to a sobbing wreck. And I may cry for Snape, because he is such a tragic character. However, I still do not like Severus Snape. [Feel free to take a minute here to exclaim over my blasphemous words. I'll wait.]
Now, most of my dislike for Snape comes from the same reason that the fandom fawns over him: his unrequited love for Lily. Because every decent thing that he ever did, every small act of redemption, was done for her. In other words, the only good Snape ever did was because of an unhealthy obsession over another man's dead wife. He was willing to let her husband and infant son die so he could have her. He spent six years mercilessly tormenting a likable boy who already has the weight of the world on his shoulders, just because he is a reminder that he lost Lily to his biggest rival.
I could go much more in depth about this, but I was beaten to the punch. Glovebox recently posted an essay titled "The Deification of Severus Snape," that you should really go read right now. She does a fantastic job explaining why Snape is not a good person just because he loved Lily - it's a wonderfully written piece that I agree with 110%.
No, what I would like to do is go back through the series and remind the legions of Snape fans why we all hated him before we read "Prince's Tale." Because he truly is a despicable character, and kudos to Jo for making us almost forget that.
Okay, let's ignore the fact that Snape is wreaking his vengeance on eleven-year-old Harry Potter. Can we talk about the fact that he traumatizes poor Neville? And what has poor Neville ever done to him? He regularly humiliates Neville in front of his peers, destroying any shreds of self-confidence that Neville has. He even goes so far as to belittle Neville in front of other teachers (Prisoner of Azkaban, 132). He picks on Neville relentlessly - the poor boy already had a tough upbringing with his frightening grandmother and a bunch of great-uncles and great-aunts who'd drop him out of windows. After everything Neville has been through - after knowing his parents were tortured into insanity by Death Eaters - the thing that he fears most of all at age thirteen is Snape.
"Right, Neville," said Professor Lupin. "First things first: what would you say is the thing that frightens you most in the world?"
Neville looked around rather wildly, as though begging someone to help him, then said, in barely more than a whisper, "Professor Snape." [POA 134-135]
Now yes, we all had a great laugh when Neville's boggart turned into Snape, and ended up dressed as Neville's grandma, and Lupin turned out to be the coolest teacher ever. But after reading Order of the Phoenix, and realizing the horrors that Neville has in his past, the comedic scene from Prisoner of Azkaban takes on a much darker meaning.
Neville's boggart should have been Bellatrix Lestrange. Or perhaps it would be losing his mind like his parents did. Or being a Squib (since we know Neville has a lot of issues with that). Or even something less serious like a mummy or a spider. Now imagine how terrified he must be of Snape, that Snape scares him more than Bellatrix. This isn't just being scared of a teacher, this is being traumatized.
Earlier in the chapter, Snape actually went so far as to poison Neville's toad, Trevor:
The end of the lesson in sight, Snape strode over to Neville, who was cowering by his cauldron.
"Everyone gather 'round," said Snape, his black eyes glittering, "and watch what happens to Longbottom's toad. If he has managed to produce a Shrinking Solution, it will shrink to a tadpole. If, as I don't doubt, he has done it wrong, his toad is likely to be poisoned." 
This is just low - attempting to poison Neville's pet. Neville doesn't have many friends, so he's very fond of Trevor. What ends up happening is that Trevor turns into a tadpole in Snape's hand. This is very symbolic - what happens to pets in the HP series usually reflects what happens to their owners, and this shows that Snape belittles Neville and makes him feel completely inadequate. It's just very fortunate that Harry comes along to turn Neville into the badass he becomes in Deathly Hallows.
While Snape's dislike for Neville can never be excused, it's at least explained by the fact that Neville is an abysmal student. Why Snape has it in for Hermione, on the other hand, is perplexing. Hermione starts off the series by being just as insecure as Neville is - shown in moments of vulnerability - she's just much better at hiding it. Whereas all the other teachers (sans Trelawney) reward Hermione for her brilliance and hard work, Snape either sneers at it or refrains from commenting when he's feeling generous. In fact, he goes so far as to punish Hermione for it.
"Five more points from Gryffindor for being an insufferable know-it-all."
Hermione went very red, put down her hand, and stared at the floor with her eyes full of tears. 
Okay, can we take a step back and think about this situation if it were real. I appeal to the adults out there: how would you feel if a teacher called your thirteen-year-old daughter an insufferable know-it-all and reduced her to tears? What kind of adult, teacher or not, would stoop to insult a child like that? This is clearly something Hermione struggles with among her classmates, who have a reason to resent her academic prowess. To be insulted for it by a teacher is probably the worst thing that could happen to her emotionally.
But Snape stoops even lower to humiliate Hermione (and Harry as well): he actually reads aloud a gossip article about them in class.
"'Harry Potter's Secret Heartache'... dear, dear, Potter, what's ailing you now? 'A boy like no other perhapsâ¦'"
Harry could feel his face burning. Snape was pausing at the end of every sentence to allow the Slytherins a hearty laugh. The article sounded ten times worse when read by Snape. Even Hermione was blushing scarlet now.
"'... Harry Potter's well-wishers must hope that, next time, he bestows his heart upon a worthier candidate.' How very touching," sneered Snape [â¦] "Well, I think I had better separate the three of you, so you can keep your minds on your potions rather than on your tangled love lives." [Goblet of Fire, 515]
This situation is so absurd I can't even pretend to consider it as a real situation. Snape is taking a slanderous gossip article that could severely hurt Hermione's reputation, and reads it aloud in his class as an open invitation for Hermione's fellow students to abuse her. This is sadism so impressive that Umbridge would be proud. Because that's really all Snape is: a sadist who psychologically destroys children to make up for his own miserable existence.
But perhaps the worst thing he does to Hermione is actually earlier in Goblet of Fire, where he mocks not her mind, nor her love life, but her appearance. To provide a bit of context, Harry and Draco try to curse each other, and their curses ricochet off each other and hit Goyle and Hermione. Goyle gets covered in boils and is sent to the hospital wing by Snape. Hermione's large teeth are now growing at an alarming rate, and Ron calls Snape's attention to this.
He forced Hermione to show Snape her teeth - she was doing her best to hide them with her hands, though this was difficult as they had now grown down past her collar. Pansy Parkinson and the other Slytherin girls were doubled up with silent giggles, pointing at Hermione from behind Snape's back.
Snape looked coldly at Hermione, then said, "I see no difference."
Hermione let out a whimper; her eyes filled with tears, she turned on her heel and ran, ran all the way up the corridor and out of sight. [299-300]
Wow. Just wow. Now, let's keep in mind that this is fifteen-year-old Hermione - before she became a confident adult, before she had Viktor Krum ask her to the Yule Ball, before anything ever happened to make her feel pretty. She is a teenage girl - and we know how vulnerable teenage girls are where their body-image and self-perception are concerned. All she knows is that she isn't very attractive, the guy she likes isn't aware that she's a girl, and no guy has ever looked twice at her.
And then an adult, a teacher, insults her appearance in front of everyone. This must be more emotionally scarring than anything else Snape ever did. And it was completely uncalled for! No one was interrupting him or reading magazines in his class, he just chose to lash out at Hermione. I'm a guy, so thankfully I never went through this, but I vividly remember my female friends' crazy diets and whatnot, and how upset they were over their less-than-perfect appearance. Poor Hermione.
Sirius and Lupin
Lest we think it only children that Snape abuses, Jo ensures that we are privy to how awfully he treats his fellow adults. Now, I will ignore Snape mocking Sirius in Order of the Phoenix, because both of them hated each other and acted like sixteen-year-old shmucks. Snape deriding Lupin in front of his class is more serious. But the truly appalling stuff doesn't happen until Snape actually has power over Sirius and Lupin, and (as usual) abuses it to the fullest. This happens when Snape stumbles onto the two of them and the Trio in the Shrieking Shack in Prisoner of Azkaban (for a very in-depth analysis of Snape's behavior and motivations in this scene, check out my essay "Snape's Anger").
"You fool," said Lupin softly. "Is a schoolboy grudge worth putting an innocent man back inside Azkaban?"
"Up to the castle?" said Snape silkily. "I don't think we need to go that far. All I have to do is call the dementors once we get out of the Willow. They'll be very pleased to see you, Black ... pleased enough to give you a little kiss, I daresay..."
"I'll drag the werewolf. Perhaps the dementors will have a kiss for him too -" [POA, 359-360]
Here, my feelings toward Snape go from contempt to alarm - he is willing to condemn two men to having their souls sucked out because one bullied him as a teenager, and the other didn't stop it. I don't know whether Snape believes they are innocent or guilty - the alarming thing is that Snape doesn't care. He doesn't care if he sentences innocent men to a fate worse than death, as long as he exacts his petty revenge for deeds done twenty years ago.
Here is where we see Snape the Death Eater at his worst - the man who will kill innocents with no regard, the sadist who abuses his power at any given opportunity, and a man whose own agenda is more important that anything else. If Voldemort had not killed Lily, I'm sure Snape would have been a top Death Eater like Bellatrix Lestrange or Malfoy.
I know that some of you are still crying that yes, Snape was deeply unpleasant at times, but he was still on the good side! Doesn't that, along with his unrequited love for Lily, prove he's a good guy.
Well... no. Just because someone is working against Voldemort does not mean he is a good guy - a wonderful example would be Cornelius Fudge. But there is proof that Snape considered working for Dumbledore a secondary goal compared to that of making Harry miserable.
Let us recall the instance in Goblet of Fire where Mr. Crouch shows up while Harry and Viktor Krum are talking, and Harry runs to get Dumbledore so the latter can attend to the insane Mr. Crouch. And while outside Dumbledore's office, Snape happens on by:
"Mr. Crouch!" Harry shouted. "From the Ministry! He's ill or something - he's in the forest, he wants to see Dumbledore! Just give me the password up to -â
"The headmaster is busy, Potter," said Snape, his thin mouth curling into an unpleasant smile.
"I've got to tell Dumbledore!" Harry yelled.
"Didn't you hear me, Potter?"
Harry could tell Snape was thoroughly enjoying himself, denying Harry the very thing he wanted when he was so panicky. [Goblet of Fire, 558]
Let's consider this. Snape has just run into a frantic Harry trying to get into Dumbledore's office (which Harry probably wouldn't do without good reason). Harry yells that a vanished Ministry official has shown up on the Hogwarts grounds and is crazy, and this high-ranking crazy Ministry official is running amok with a Triwizard champion. Snape decides to completely disregard all of this for the simple joy of tormenting Harry for a little bit.
It's a rather interesting way to prioritize. Snape ignores his duty to Dumbledore and whatever is happening in the wide wizarding world, all to pettily spite a fourteen-year-old. It's really just pathetic. So Snape is on Dumbledore's side... when it suits him.
So after remembering all of these choice Snape moments (and I'm sure there are plenty more, and we have only scratched the surface of what he's done to Harry), I just can't think of him as a good guy. Yes, Severus Snape was technically on Dumbledore's side, and he has a beautifully tragic unrequited love for Lily Potter. However, he is still an awful, spiteful, petty, vindictive, and cruel human being. He is not fit to interact with children, much less teach them.
So while I love Snape as a character, and think of him as one of Jo's crowning literary achievements, I still hate him as a person. And perhaps the fangirling would be better directed towards Alan Rickman.
That'll change the world, that report will. Front page of the Daily Prophet, I expect, cauldron leaks.
Ron Weasley Goblet of Fire, Chapter 5, Page 56
Natalie McDonald, who appeared in Harry Potter and Goblet of Fire, was based on a real girl Rowling knew who was dying of leukemia. She wrote to JK Rowling asking what was going to happen in the next Harry Potter book as she would not live long enough to read it.