Snape was never a good guy though. Very brave, yes and he had some good qualities. He was also vindictive and a bully - willing to put his petty dislikes above the quality of his teaching.
The book strongly implies that Snape turns into a horrible person as a direct consequence of James Potter's bullying. He seems to be a nice kid before that.
In the scheme of the books, yes. He was a main antagonist for a while. In terms of his character, to what end did he have to act like a total dick to a child? No, he's just a salty bitch.