I doubt few would disagree with what Helen's examples illustrate: on the level of mundane daily interpersonal living, Snape could be mean and unfair, and abuse his authority as a teacher.
By the time Bk7 concluded, we all understood a lot more about the childhood privations and neglect/abuse, and the horrible bullying from James and Sirius during a chunk of his Hogwarts years. Yet admirable character isn't built by hiding behind the past as excuse-justification, but by rising above past (and, often, present) circumstances instead of devolving to the lowest common denominator.
It's often the case that those who succeed or sacrifice on the larger stages of life -- such as risking one's life to save the wizarding world from torture, lies, enslavement, et al. -- can be downright ignoble on the level of how they treat some or most people . . .
. . . Sirius, for example, could both remind Harry that one can best judge a person by watching how they treat those of lower, rather than higher rank, and nevertheless treat Kreatcher shabbily out of resentment toward his own family -- only to have that become the means by which Voldemort obtains the info to lure Harry and Sirius into a situation that kills Sirius.
And Snape simply is damaged. We can admire his sacrifice and the skill and intense discipline that required, but think poorly of a lot else about his behavior, including, maybe, the obsessive-possessive (needy?) way that -- perhaps -- he loved Lilly.
But here's something that is too easily overlooked: being strong in the face of crappy treatment, like what Snape dished out to Harry, is a hugely valuable life skill -- and Snape knew Harry would need to be tough in many ways in order to succeed in whatever his special task would become.
And, no, I don't believe Snape's bullying and unfairness was an act soley for the purpose of toughening up Harry.
I do believe he was loyal to Lilly and his promise to Dumbledore, and impressive in his endeavors to do the dangerous work that he performed nobly -- and was inexcusably unpleasant in certain of his interpersonal relations and abusive of his authority as a teacher.
But he did suffer remorse and perform the ultimate in restitution to redeem his early association with evil. That is profoundly admirable.