Random thought
I was thinking that considering what Sirius's family was like that that might just be the crux of why Snape was singled out for abuse.
In an attempt to distance himself from his, Sirius's, family and their own dark affiliations he used his public torment of Snape. Basically standing up and saying "No, I'm not like that. Would I torment him if I was?"
I never said it was a deep thought, but a thought nonetheless.