One blink can tip the balance between good and evil. Owen learned this in the Torchwood medical room. One blink and he had slipped from being Owen Harper into a demon, filled with darkness and beautiful rage.
Even though he couldn't remember, he knew as soon as he saw the others that something was wrong. Gwen's sad eyes, Tosh's total professionalism, Jack's tortured frown and Ianto's pitying face.
He just didn't know why yet.