God's Law is so important - in fact crucial - in showing us our need for the cross of Jesus Christ and his payment for our sins. When witnessing it is so important to present the Law, for otherwise the Gospel is a cure for an illness most don't even know they have. "For the message of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God." (1 Corinthians 1:18). Presenting the law makes the Gospel necessary, truly showing the love of the Father.
The ten commandments can be a simple way of imparting God's Law to unbelievers. Most think they are a good person. Go ahead, ask ten of your unbelieving friends if they think they are good (unfortunately your saved friends may give the same answer). Nine out of ten will likely say they are good. Go through the commandments with them, show them the measure isn't whether or not they are good when compared to fellow man, it is whether or not they are good according to God's standards, the moral Law, embodied in the ten commandments.
God will judge them on that last day - will they be guilty? A just judge would find a criminal guilty, and God IS just, and will provide justice through punishment in hell. Eternity in hell is a just punishment for crimes made against an infinite God. The Good News is that He provided a way for all to have their sentence fulfilled - the ability to come to him washed clean in the saving blood of His son Jesus Christ. Each person has to come to this realization that they have sinned against a just and holy God. They can exercise their free will by asking God to save them from the wrath to come, repenting of sin and trusting in Jesus as their only savior, the only one provided by the Father. The Holy Spirit will work faith in them, and they will receive salvation.
Mirror, Mirror on the wall, who is the biggest sinner of all?