to make someone take an unbreakable vow.

However, I do believe Snape's good.

All evidence points to it. Otherwise there's no reason that he'd be so frustrated that Harry was so lazy. He'd be HAPPY AS SHIT, cuz that means an automatic W for Voldemort.

Also, he never woulda hesitated to take the vow with Mrs. Malfoy.

Also, pay close attention to the scene where Harry is running after Snape across the school grounds. He's talking shit to Harry as usual, but he's very specific in the shit he's talking. He's making a last-ditch effort to get the point across to Harry that he needs to take everything Snape was trying to teach him seriously, because if he can't beat Snape he'll never beat Voldemort.

The ONLY thing he takes offense to in that scene is when Harry calls him a coward.

SHIT, if I was undercover on Voldemort's squad I'd be pissed if someone called me a coward, too.