"He told me." Severus kept his place on the bench, but his hand tightened around his wand. "Sit down. It was not a lie; Albus Dumbledore did not lie to me." He simply ... omitted. "I killed him because he asked me to. I will explain to you as much as I can, but you must at least try to look as though you're having a normal conversation. I've been working to secure the Dark Lord's downfall for the better part of twenty years - and as much as I need your help, I cannot take on a liability. You are obviously capable of discretion. Exercise it."