On top of that, Potter was somehow made the Dark Lord's heir (likely through right of conquest, or something along those lines. Severus would have to ask Lucius to be certain. The Malfoy lord knew everything about lord- and heirships.), which would explain the changes in Potter's behavior. Severus also found out what exactly Draco's mission was. Draco had been tasked with finding a way to get Death Eaters into Hogwarts without the headmaster or other professors knowing, and Potter had asked to be the one to end Dumbledore's life.