The Mekhilta DeRabbi Yishmael uses a vivid parable to explain why murder is equated with diminishing the divine image. The teaching compares God to a king of flesh and blood who enters a new province and does what any sovereign does: he sets up statues of himself, commissions images in his likeness, and mints coins bearing his face. His image is everywhere, a symbol of his authority and presence.
Then, after some time, the people turn against him. They topple his statues. They smash his images. They devalue his coins. With each act of destruction, they diminish the likeness of the king. The king himself is not physically harmed, but his representation in the world is erased piece by piece.
The parable maps directly onto the prohibition against murder. God created human beings in His image, as stated in (Genesis 9:6): "One who spills the blood of man... for in the image of God did He make man." Every human being is a living representation of God in the world, just as the king's statues represent the king in his province.
When someone murders another person, they are not merely ending a life. They are destroying a living image of God. They are doing to God what the rebels in the parable did to their king: smashing His likeness, diminishing His presence in the world.
The teaching elevates murder from a crime against a person to a crime against the divine image itself. Every human face is a coin minted by God, and to destroy it is to deface the currency of creation.