Look, Constantine was a complex man. He also was no saint, though a saint. You don't become sole Emperor, overthrow the Tetrarchy and maintain your rule, without having quite a lot of blood on your hands. He facilitated Christian processes that were already underway in potentia, like the Council of Nicaea. The Empire only 'went Christian' much later with Theodosius, and throughout the period of the Flavian Emperors (Constantine and Successors), many of the pagan structures went on as before, with varying levels of support or withdrawal thereof, by the Emperors. We still see prominent Pagans becoming proconsuls of Africa and such, as late as the fifth century.
Constantine is an important figure in Christianity, and certainly had an effect in moulding Roman practice toward it, but this story of Constantine corrupting Christianity or changing it, really has little basis. If that were the case, why did his openly Arian successors like Constantius II, fail so miserably? It just doesn't pass muster.