There are two traitors among Voldemort. Peter Pettigrew and Severus Snape. The former the reason Lily is dead. And Severus Snape must act as if that thought pleases him.
Each on their first night at Hogwarts, Harry's three children visit the headmaster's office to visit the portraits. Two in particular.
After the fateful Mudblood incident, Lily is sitting outside when Severus Snape walks out. A chance to repair the mistake that was made?