Snape did some good things. But did those good things make him a hero? It is yet to be decided.
Severus Snape was a hero
Snape saved Harry's Life in the first HP book. He also spied on the dark side for Dumbledore. Not to mention he loved Lilly, a true villain has no love in his heart. Even if Snape wasn't your average hero, he did much good.
Snape saved Harry in the Quidditch Match in The Philosopher's Stone.
When Quirrell was trying to make Harry Potter fall off his broom in The Philosophers Stone, Snape tried to use a counter-curse to stop Quirrell. Hermione and Ron thought at the time that it was Snape trying to curse Harry.
He was a bully of a teacher to all of his students. (Except Slytherins) He also joined the Death Eaters when he was younger. A true hero wouldn't do either of those things. Even if Snape did some good things, a true hero wouldn't have been that cruel.
Snape joined the Death Eaters
When Snape was a late teen, he joined the death eaters. (Later on, he spies for Dumbledore) He does this because he wanted to. He was not forced. He chose to.