Their conflicting thoughts accuse or even excuse them.

Romans 2:14-16

14 For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law.
15 They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them
16 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus. – Romans 2:14-16

We have an inbuilt sence is justice. If someone steals our phone we rightly want them to get caught and punished. But why? In the animal kingdoms there are no police, no judges, no courts and yet authority, “pecking orders” and harmony are maintained through instinct and learned behaviour. His has set an order that is maintained to ensure the animal kingdoms remain. (Most extinctions are due to mans interference.) But we are different. We have been made in the image of God and are built to be relational. We need to be in relationships. God said “It is not good for man to be alone” (Genesis 2:18). We are designed not only to have a relationship with other humans but primarily with God. And within those relationship we have to have rules written on our hearts. Jesus said:

37 And he said to him, “You shall love the Lord your God with all your heart and with all your soul and with all your mind.
38 This is the great and first commandment.
39 And a second is like it: You shall love your neighbor as yourself.
40 On these two commandments depend all the Law and the Prophets.” – Matthew 22:37-40

All the law is summed up in this. Love God and love your neighbour. If we all submitted to God with humility and if we all loved each other as we love ourselves there would be no need for a judicial system, no need for a defence budget, we would live in peace.

Sadly that is not the case and for now we are all under the judgment of God as we all sin. The law may be written on our hearts, we understand what is right and wrong and yet we still do wrong. And then we either accuse or excuse ourselves in our minds “oh I am a terrible person” or “well everybody does it”. Whichever way unless we face the truth and admit our guilt before God we will never be free from the consequences of our actions. Every thing that is done in secret is known by God and will be revealed, we have no excuse. Will we see a complete re-run of our lives when we stand before God? I don’t know but what I do know is that I am ashamed of my past as most people who are honest which their conscience are. Thank God that he has made a way to be freed from the punishment we deserve through the shed blood of Jesus at the cross.

9 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved.
10 For with the heart one believes and is justified, and with the mouth one confesses and is saved. – Romans 10:9-10