The Jesuits made up the claim Nero was the "666" in order to counter the claims of the Reformers saying the Papacy was the "666".
Nero did commit incest with his mother, indicating he may have done so during a Mystery Cult ritual wherein the "mother" is both the mother and consort of the reincarnated god because that god died yearly and had to create his new reincarnation by impregnating the same goddess who gave birth to him.
Whether his involvement in such a ritual included human sacrifice, is not determined. How this ritual was carried out depended upon the culture enacting it.