Snape has always been one of my favorite characters because of his complexity - he's not a nice man, he does things that are extraordinarily nasty, and yet he clearly has a strong sense of honor. I wanted to comment on his backstory from OotP. One thing that's particularly striking abou the Pensieve scene with Harry is how *controlled* he is throughout. When contrasted with his rage when he *truly* loses his temper (ie with the Sirius incident), one has to suspect that he set up the carefully selected scene for Harry's viewing pleasure. The question is why - but I'm not certain. The obvious one is to protect his cover from Lord Voldemort - if the Occlumency lessons weren't successful, then perhaps he needed to cut them off before LV could see from Harry what Snape was doing. Whatever the reason - I think JKR used it less as a ploy for us to feel sympathetic to Snape, and more as a plot device.