Why am I guilty?
I am guilty because I am not perfect. God did not give man the Ten Commandments to tell us to try harder to be good, but to show us we can’t be good enough. He is morally perfect and only a life that is morally perfect is considered morally good in His eyes. (No one is good, except God alone (Mark 10:18)).
This is how He measures moral perfection: if I break one commandment, I am as guilty as I would be if I broke all of them (James 2:10-11). It is all or nothing. I keep all or I keep none. There is no in-between. The issue is not whether my good deeds outweigh my bad deeds. The issue is this: do I have one bad deed? If so, I am considered a transgressor (lawbreaker) and I am guilty of breaking all the commandments.
One additional truth drives home even further the helplessness of my situation. The commandments must be kept from the heart. Lust is committing adultery from the heart. Hate is committing murder from the heart (Matthew 5:21-28). Although I may restrain my outward actions (murder, adultery), my inner thoughts (hatred, lust) are not so easily controlled.
An x-ray exposes a problem and I see I need a doctor. The law exposes my sin and I see I need a savior. It convinces me of my guilt before God and lays me at the feet of Christ.