Hi Snowbird,
I think you will find that other Roman emperors were responsible for persecuting Christians.
Constantine had a dream (vision) which included a Christian symbol. He said that if he won the battle against his enemy the following day, he would become a Christian. In reality, he was not baptised until he was on his death bed - and the ceremony was performed by an Arian Christian (not by a Trinitarian).
It is absolutely my pure guess and assumption that Constantine's support for Christianity was deeply influenced by his mother Helena (Helen). Read the article at Wiki about her:
https://en.wikipedia.org/wiki/Helena_(empress)
As emperor, Constantine's main interest lay in maintaining peace throughout the Empire, so the dissensions within the eastern part of Christianity deeply troubled him, especially as he could not understand why the eastern churches were embroiled in the minor matter of the nature of God and of Jesus Christ.
When he convened the Council of Nicaea, hardly any of its delegates came from the western church, and the Bishop of Rome played no part in it or in its outcomes.
Doug