Our sin requires that we be separate from God because his glorious holiness would destroy us.  Even our "no big deal" sins actually violate several of the 10 commandments by setting us up as gods ahead of the true God, lying about his character and misrepresenting him in a way that takes his name in vain.  Sin incurs a debt for which God's justice demands payment.  The price is death symbolized by blood.  God instituted animal sacrifices which ultimately point to the perfect atoning sacrifice of Jesus.