People will say that Snape was a bad guy because he joined Voldemort...

But I think this was something different. If you consider what we know about Severus childhood, he had no friends. Once Lily wrote him off he was essentially alone. We know that Lucius Malfoy is the one who introduced him to Voldemort, but we don't have the specifics. I feel like Severus joined the Death Eaters because he wanted to fit in. He wanted a group who would have his back no matter what. Once he realized what Voldemort was really doing he tried to get out, but couldn't.

Some might say Snape is the reason Lily is dead...

I disagree. I feel like even if Snape hadn't told Voldemort the bit of prophecy he heard, the Potters still would have been attacked. They were known loyal members of the Order of the Phoenix and if you remember, Voldemort was attacking known Order members trying to figure out what Dumbledore knew and what the Order's plans were. Lily and James would have been attacked eventually either way. They may have lived, but they may have ended up like Neville's parents, which I personally feel is worse than death.

People might also say Snape was a bully for harassing Neville Longbottom... And Hermione Granger... and Harry Potter...

I feel like there are reasons for this though, that while not making up for Snape's actions, do explain them.

Neville was one of the two who, in the prophecy could have been the "boy who lived" had Voldemort chosen differently. I feel like Snape was trying to make Neville stronger by always being on his case. If Neville could survive day after day of Snape's torture, he might just survive the upcoming battle.

With Hermione, Snape told her she was a know-it-all once in front of the whole class. He even insulted her teeth size after a jinx had gone wrong. I feel like Severus was trying to show her that not everything can be learned from a book. That sometimes things in life go badly and you have to make a choice about how to handle them.

Snape's grudge against Harry is pretty self-explanatory. Since everyone said Harry looked mostly like his father, Severus was stuck seeing James Potter, who tortured Severus for years, all the time. Would you be totally cool with seeing the person who married your only love every single day for years on end? No! You wouldn't. I also feel like Snape knew that if he was super nice and friendly with Harry the Death Eaters' kids would report back to their parents. Can you imagine Draco complaining to Lucius that "Severus was nice to Potter today," and it getting back to Voldemort? Yikes!

The other argument is Snape killing Dumbledore makes him the villain...

I feel Snape had no choice here. Let's say for argument's sake that Snape ignored the dying man's wish and didn't kill him. If things still happened in the say way as they did without Severus making the final blow, he would have been ousted as a traitor much sooner. Bellatrix or another Death Eater would have killed Dumbledore and Snape would be dead too. Because of his unbreakable vow to Narcissa Malfoy to help Draco and completed his task, of killing Dumbledore, if Draco could not, Snape would be dead without following through. So, if Snape is dead that means in Deathly Hallows, the Carrows are running Hogwarts, torturing all the students. And Harry has no clue he's a Horcrux and needs to die because there is no Snape to give him the information. Which essentially means Voldemort wins because he can come back through the piece of his soul in Harry.