There is a lot going on in an individuals heart before committing the act of physically murdering another human being; namely, as Jesus points out here: anger and hatred are deeply present.

God’s focus is always primarily the heart behind our actions. To be angry with another; to insult another; to treat them with contempt; to say of another ‘you fool’ condemns us. When we have this attitude, It is us entertaining the root of murder in our heart, because the root of murder is anger.