I thought that he had always had a dislike for someone who wasn't born with magical powers. You can see it when he's talking to Lily about Petunia.Losing Lily might have brought about a change of heart in him, kind of like losing Ariana did the same for Dumbledore, who lost his desire to rule 'for the greater good', and made him kinder to non-magic folks and magical creatures.
Well, there is the scene during Snapes memory when Phineas says mudblood (speaking of hearing Hermione) and there is a sharp "Don't use that word." Whether it came from Snape or Dumbledore's portrait - it didn't say but if it came from Snape then you could definitely see his new tune there.
I remember Dumbledore asked Snape if he would promise to keep the students safe and yet he was allowing the Cruciatus Curse to be used as punishment.