Republic Book Two

In the Republic Book two Glaucon discusses his idea that if being unjust had no reprecussions everyone would choose to act unjustly. This implies that humans are naturally bad as opposed to naturally good people that crave justice. If this is true, than the idea of Eutopia that was brought up later in Book two could never exisit because people are not only imperfect, but they are naturally and undeniably evil. It confuses me how we have no example of what perfection is but yet we have the idea that it could exisit.