Very interesting talking to you, too!
Maybe the word sadist is what's throwing me off. To me, a sadist seeks to hurt others simply because he (or she) enjoys and derives pleasure from hurting other people. To me, someone who's hurt or angry or frustrated (or beset by many complex emotions) and strikes out because they act on their emotions is not a sadist. I've always understood a sadist as being detached emotionally from the situation, meaning they don't seek out people who have harmed them and they don't act rashly or impulsively. It's planned and deliberate. And I don't believe Snape falls in that category, although he might have that tendency simply because he must repress and control his emotions when he is around Voldemort.
His treatment of Harry is based on his feelings about Harry (and James), whether those are misplaced or not. At a very basic level, he's angry at Harry because James took Lily away from him. Is it logical or rational, no, but it's not sadistic. He truly is one of the most tortured characters in the series. Very complex, yet one of her most real characters, terribly flawed, but still with admirable qualities. He's very believable.