This unsub is a classic picquerist. Someone who uses a knife to achieve secondary or indirect sexual release. Picquerism is the act of stabbing or cutting, any repeated penetration of the skin with a sharp object. The knife is a phallic symbol--a substitution for the male sexual organ. Instead of performing normal sexual intercourse, our unsub achieves his release by subjecting his victim to pain and terror. It's the power that thrills him. Ultimate power, over life and death.