Webster's Dictionary defines justice as the quality of being just; conformity to the principles of righteousness and rectitude in all things; strict performance of moral obligations; practical conformity to human or divine law; integrity in the dealings of men with each other; rectitude; equity; uprightness. .
Thrasymachus's views justice as an advantage of the stronger. The example he gives I that the government makes up laws to help itself and declares just for their people. This proves Thrasymuchus's point in that justice benefits other people while injustice benefits you. In modern day we can take Robin Hood for example, he helps himself and his people by stealing from the rich and giving to the pour. But after overcoming the king he wins the love of his princess. If you are Spider Man and help everyone else you are just, but you still are pour and don't have Mary Jain. .
The invisible ring is one way that living just is better then living unjust can be proven. If you are never seen committing the crime is it a crime. If I did some insider trading on the stock market I could make billions. Who would this harm? Is this unjust? Well if I take twenty of the smartest doctors in the world and pay them to work in a lab until they create a cure for cancer will be man of the century. If there are any gods they this will wash all my sins. With all the money I have I can buy a house, cars, get married and have a happy life, as an unjust man. .
Unjust people have themselves in mind, but on the way they can help the pour or the sick. This will only help you if there are gods. If so they will forgive you for helping the world and you will live life to its fullest, and if there aren't any gods then you still everything you need. This is why being injustice is better then justice. .