Why Is Plato Better Than Being Unjust

In book II of Plato’s Republic, there is a discussion between Glaucon, Adeimantus, and Socrates. Glaucon and Adeimantus states that justice belongs to the class of goods, which we welcome for its own sake and for its consequences. Glaucon, then, picks Thrasymachos’ argument, that is better to live an unjust life than just life. In this dialogue, Glaucon challenges Plato to proof why be just is better than being unjust. For Glaucon human being is by nature competitive, for Plato human nature is either good or bad.
In real life, real world some people are born bad and other are born good by nature. In Plato’s idealistic world, with proper training this nature could be turned good. I would have to disagree with Plato’s teaching. Take the white