If it can be demonstrated that Constantine did something more gruesome than culling a nation's population down to a third of its size and authorizing a genocidal campaign against another nation, then I will consider calling him a tyrant.
Israel Finkelstein said," David for me is the David reflected in the later king Hezekiah, the David of Zacharias in the eschatological prophesies in which Jerusalem is burned but David is alive, the David who is the connection with the beginning of Christianity. In this sense, David is everything. If you want me to say it simplistically, I'm proud that this nobody from nowhere became the center of Western tradition." (National Geographic, December 2010, p.90)
If Finkelstein can say such things of the biblical David, I feel no guilt in a few high accolades for Rome's first Christian emperor.