As we all probably know by now, this is Severus Snape.
Well, I'm here to educate you all. Severus Snape is merely a jackass who had a few redeeming moments.
First off, a bit of background info for those who aren't totally down with the storyline:
Snape was interested in the Dark Arts from a young age. We see this in flashbacks to his childhood when he was friends with Lily. As he progressed through his years at Hogwarts, he became more and more involved with pure-blood supremacists, despite his own half-blood status. He stopped being friends with Lily after calling her a Mudblood (huge insult), and became close friends with students who later became Death Eaters.
After graduating from Hogwarts, Snape joined Voldemort and became a Death Eater in the First Wizarding War.
He learned through spying and eavesdropping on Dumbledore that someone with the power to vanquish the Dark Lord would soon be born. Snape told Voldemort, who in turned assumed the child would be born to Lily Evans. THIS WAS SNAPE'S TURNING POINT. His eternal love for Lily caused him to DOUBLE CROSS VOLDEMORT and ask Dumbledore to save Lily. Snape then spent the rest of his life working as a double agent, providing Dumbledore with information about Voldemort's plans.
It is certainly undeniable that Snape risked his life for many years, working with Dumbledore behind Voldemort's back. I AM NOT DENYING THIS. BUT IT DOES NOT MAKE HIM A HERO.
THE ONLY REASON HE DID THIS WAS BECAUSE HE WAS PATHETICALLY IN LOVE WITH A GIRL WHO DUMPED HIM BECAUSE HE WAS A DICK TO HER.
If Voldemort had assumed that Neville were the Chosen One, not Harry, Snape WOULD NOT HAVE CARED AT ALL. He would have continued on with his Death Eater life, killing Muggles by day, weeping over Lily by night. He would have done nothing worthy with his life and we wouldn't even be having this discussion.
For the first six years we knew him, Snape was continually cruel to his students, for no apparent reason other than he's a bitter, lonely old man.
Remember when he forced Neville to feed his toad a potion that was potentially poison? Remember how Neville's boggart was Snape, because he was SO AFRAID OF HIS TEACHER? How about when he made fun of Hermione's large front teeth? He constantly called his students idiots. He tried to 'out' Lupin as a werewolf when he was teaching at Hogwarts. There was also every single potions lesson where he showed extreme favouritism for the Slytherins.
Yes, Snape was brave. Very brave. But he in no way was a hero.
His work with the Order of the Phoenix was brave, noble and worthy. But in no way did it make up for his lifetime of bad deeds. Before he became "good", he was a pure-blood supremacist, killing and torturing people on Voldemort's behalf. He ruined his relationship with Lily through his own bigotry, and spent the rest of his life taking out his pain on everyone around him. Yes, he was bullied at school. But this is no excuse for the way he turned out.