His propensity for good has been spotted in a Harry Potter and the Deathly Hallow Part II movie scene.
In the film, Snape is forced to fight Professor McGonagall when Harry Potter reveals that he was the one that killed Professor Dumbledore (at his bidding, but they don't know that).
Snape defends himself against the witch's spells without firing off any of his own despite the fact that he was the Defence Against the Dark Arts teacher, meaning he was more than capable of fighting, and probably winning against her.
Not only does he refuse to fight back, but it appears that he deliberately deflected one of her spells to hit the two Death Eaters standing behind him, before flying away.
He made sure they were safe before he left.
Picture: Warner Bros
If the blubbering mess in the corner is you, it's okay.