From the very beginning Satan has been the chief instigator of evil in the world… his two main characteristics being lying & murder. In the Garden of Eden the serpent twisted the truth & caused Eve to doubt God, and later Cain became envious of his brother Abel’s offering and murdered him. And thus Satan’s reign of terror began on earth. So as Jesus is telling us in this verse, we have two options…we are either children of God or children of Satan. If our inclination & impulsive urges are unrighteous, deceptive, and false…we follow after our father the devil. But as I John 3:8 says, if we do what is right & love one another, we are children of God. Satan’s original language is lies…the total opposite of God’s truth, and murder is not just being guilty of killing someone. for both Galatians 5:19-21 and Matthew 5:21-22 both speak of the evil works of the flesh & specifically Matthew 5 show us that anger & hatred towards a brother is considered murder in God’s eyes & thus subject to judgment.