How serious is the willful taking of a human life for a reason not authorized by God?

How serious is the willful taking of a human life for a reason not authorized by God?

Gen. 9:6: "Anyone shedding man's blood, by man will his own blood be shed, for in God's image he made man."

1 John 3:15: "No manslayer has everlasting life remaining in him."

Ex. 20:13: "You must not murder."