"...if you do not do well, sin is crouching at the door. Its desire is contrary to you, but you must rule over it.” - Genesis 4:7
Cain failed to safeguard against sin in his anger and murdered his brother Abel. His line apparently fostered that violence, with his descendant Tubal-cain forging all instruments of bronze and iron (weaponry?) and Lamech declaring to his wives that he killed a man.
(Genesis 4)
Murder defiles a person
Jesus teaches murder defiles a person, along with any evil proceeding from the heart out of the mouth. "For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander. These are what defile a person."