I was reading The Ten Commandments - Manual for the Christian Life, By J. Douma, when I came across the statement found in the title^. Douma's commentary is to the point. He goes through Exodus 20, digging up the text and explaining the intricacies of each commandment.
Thus, my light bulb moment:
"We may certainly deduce from the negative formulations that man is a sinner inclined to transgress the law. In the law we have a mirror in which we look to see ourselves as we really are: you shall not kill, because you are a murderer; you shall not commit adultery, because you really are an adulterer! It is absolutely necessary that we come to know the law in this unmasking function (usus elenchticus)...the law of God liberates us from conceit and self-righteousness. It tells us rather clearly that we had better not look for our salvation in being decent or moral or law-abiding, but that we can be declared righteous only through a living bond with Christ - who has fulfilled the whole law.(10)"