There's a lot of dispute over why Constantine chose Byzantium as the new capital, but the general population seems to agree that it was likely because Byzantium was more easily defensible against attackers. The Balkans on the West proved difficult for attackers to pass over. Also the city was located on the water which not only allowed for trade via ships, it also allowed for naval defense.
It was justified because he did what anyone else with the opportunity would do. His businesses practices were creating monopolies and trusts but what he was doing wasn't illegal up to a certain point in time when the courts decided to intervene. They all did it and they all became immeasurably wealthy. The government responded by introducing new laws to prevent such trusts and the labor was unsatisfied because they controlled everything.
There might be an increase in food demand.
The cathedrals using Gothic architecture.
