I imagine Snape becomes a very different character, with different motivations than we originally saw, in a second read through, knowing how he felt about Lily, etc. (Still excuses nothing)

Like how in Snape's Worst Memory we originally thought it was the worst because he was publically humiliated and all that (at least I did), but with some hindsight, we can see that really its because that was when he lost his only friend by being an absolute dick to her. :3