Regarding the Mudblood issue also, I know many people consider Snape's "Don't say the word Mudblood here" line as proving that he'd really changed etc., but there too I thought he had personal reason to no longer like the world--it's the word that lost him Lily's friendship. It's not that I want to try to make it seem like Snape never did anything good, this is just how the character came across to me--as somebody who did really impressive things etc., but his transformation seemed very limited to me.