We are told that Constantine was the first Roman emperor to stop Christian persecutions and to legalize Christianity. It is not stressed that was along with all other religions and cults in the Roman Empire. In 313, Constantine developed the Edict of Milan. The edict stated that Christians should be allowed to follow the faith without oppression. This removed penalties for professing Christianity, under which many had been martyred previously, and returned confiscated Church property. However, the edict protected from religious persecution not only Christians but all religions, allowing anyone to worship whichever deity they chose.
Then in 326, Constantine had his eldest son Crispus seized and put to death by poison, and his wife, the Empress Fausta, killed at the behest of his mother, Helena. Fausta was left to die in an over-heated bath.
So why exactly do we honor him so much as to call him St. Constantine? Further reading seems to indicate he was an opportunist who wanted to stop all the early inter-religious turmoil by allowing them all equal footing.