In the Pensieve's Keep

Just away from Potter's prying eyes the Pensieve held some of Snape's most scarring memories. A riding accident that demands an unthinkable choice, the first brutal ride to Hogwarts, an incident with a DE's daughter that reveals Dumbledore's bias, a family tragedy for which he is blamed . . . in short, the makings of a Death Eater.

Severus Snape had always assumed that his fall to darkness had shown Dumbledore the danger of his house bigotry. An incident endangering the life of one of Severus's students, hoever, shows just how little has changed.