Severus Snape! A bully or a hero?

Hi Potterheads,
I'm a die hard fan of Harry Potter books and of course movies. The whole series has a depth that no one could really understand and one of the main game-changing characters was Severus Snape, the Potions Master of Hogwarts. Many readers including me consider him an unsung hero. while others slander him as a bully for his foul treatment of Harry. Is it fair?

The reason why I love Severus is because of his sacrifice, his willingness to live in hell(with Voldemort) just to repent for the fault he made, JUST TO MAKE THINGS RIGHT! He lived his whole life as a double agent just to save Harry-the son of his love and his enemy. Harry's father James tormented Severus in his childhood, which pushed him towards Voldemort. He was a victim of bullying and that made him walk a dark path all his life. Even though he hated James, and even though Harry looks exactly like his childhood enemy, he protected Harry with his life. He played the traitor until the last minute to keep the plan perfect. He made everyone hate him, just to make people believe. He did everything he could, but people call him a BULLY!

Why? what do you think of him? Is he a hero or a sadist?.