2.14 "Born Under a Bad Sign"

After Sam goes missing for over a week, Dean finally receives a phone call from his brother. Dean finds Sam in a hotel room, covered in blood and suffering from the effects of a week long memory blackout. Upon investigation, the brothers find a video-tape of Sam killing another hunter in cold blood, which leads Sam to believe his dark side is taking over. When the dark side finally emerges Sam attacks Dean and continues on his hunter killing spree with his next targets being family friends Jo Harvelle and Bobby Singer.