Constantine's reign lasted from 306 to 337 A.D. but the movie goes as far as the Battle of the Milvian Bridge in 312 A.D. when Constantine goes to war with emperor Maxentius one of the other tetrarchs. As a reminder, Constantine went to war with all the other tetrachs and took full control of the Roman Empire. He also went to war with the Franks and the Alamanni in 306-308 A.D..
Maxentius was fished out of the Tiber river, decapitated and his head paraded through the streets of Rome (and later sent to Carthage so that there is no more resistance there). The movie actually contains quite many graphic images like the feeding of Christians to lions maybe to make a point about Rome's brutality before its conversion to Christianity.
When Constantine entered Rome, there was jubilation. He ended his procession at the Temple of Jupiter Capitolinus where a sacrifice is usually offered to the Roman god, a sign of Constantine's Christian beliefs. Constantine did not kill all of Maxentius supporters (which is what would be expected from a Roman emperor/general). However, he removed forever the Praetorian Guard and the Imperial Horse Guard and cancelled all legislation passed under Maxentus.