Snape was brave..I'd give him that..in fact he probably was one of the bravest people in the book. I mean changing sides isn't easy. Loving someone enough to ask your merciless master to spare them is brave!! I mean..he definitely had the guts. Loving someone despite the different paths you tread on is indeed courageous.
However, Snape made all the wrong choices. That doesn't necessarily make him a bad person. Yes, he was brave to love Lily through everything, but honestly, had he truly been been as devoted to her from the start (the time they were at hogwarts) as he was by the end of the series, he wouldn't have become a death eater. I personally believe that love brings out the best in you. If it doesn't, it isn't love. And his love for Lily brought out the best in him..however too late..During their final years at Hogwarts..he may have had feelings for her, I agree..if you say her choosing James over him was what steered him to the darkside then I would probably call that obsession..or depression..not love.
Furthermore, if you think of it from James' view point, you can clearly see that he adored Lily enough to stop all his pranking and hexing. True, he used to be a bully. True, he was probably 'an arrogant bullying toerag'..but he stopped by the seventh year..heck he became Head Boy!! And Lily's love brought out that best in him..and even IF Lily had chosen Snape over James..I really don't think James would have handed himself over to the darkside..he still would've continued fighting against it..
In my opinion was selfish..but who isn't?? In the memory, when he talks to Dumbledore and asks him to hide the Potters..Dumbledore replies that Snape surely must have asked Voldermort to spare Lily..Which Snape agrees to!! That was Snape's selfish love..he didn't pause to think how Lily would feel if her family was dead and she was alive..he didn't pause to think what she'd go through to see her husband's dead body in front of her..If Lily hadn't died for Harry..do you really think Snape would have protected Harry through his years at Hogwarts?
Plus..agreed he hated Harry who was a constact reminder of how the love of his life chose his enemy.. but what had Ron and Hermione done? What had Neville done? How was it Neville's fault that HE wasn't the one with a scar? So Snape had been bullied..and became a bully later on..
Everything I've said probably points to the fact that Snape is a bad guy..yet to me it never seems so. You can't really blame him completely for half the things he's done..so I feel that he was just a generally 'sad' character..and then again..'its our choices that determine what kind of people we are'..and he made wrong choices and wrong decisions..over and over again..so as much as I hate to admit it..interesting as he was..I don't think he realllyy was a positive character..though I still wouldn't call him bad..considering he did try to make amends..