That is a strange thing, since even one little sin is so serious that it is enough to send us to hell for all eternity. For whoever keeps the whole law and yet stumbles at just one point is guilty of breaking all of it. On Judgment Day, God as a perfect judge, must rule on any sin that we commit, so even one lie will send you off to the Lake of Fire for the rest of forever. One act of adultery will fry you. Hatred separates you from all that God is and all that God has made.
Sin is so serious because there is no getting away with it.
You can fool a lot of people about your behavior, but you can never fool God. So most of the Bible is dedicated to discussing what sin is, how it affects our lives, how we should repent of it, and finally, how we can be free from the power of it. God knew what He was doing when He wrote the ten commandments. Here Moses has just finished speaking with God. He has given ten great statutes that are to stand forever as His moral law for all of us.