The name begins as a real, local, rival god. Baal-zebub, "lord of the flies," was the deity of Ekron, one of the five Philistine cities, and its oracle was famous enough that Israel's own king Ahaziah sent messengers there to ask "whether I shall recover of this sickness" (2 Kings 1:2), the very inquiry the prophet Elijah confronts as a covenant betrayal: is there no God in Israel, that you must ask a fly-god of Ekron? By the first century the name has drifted and darkened.
The Gospels record the Pharisees pressing the charge that Jesus "doth not cast out demons, but by Beelzebub the prince of the demons" (Matthew 12:24; Mark 3:22), no longer a Philistine oracle but "the prince of the demons" outright, the title Jesus answers with the household-divided argument (Matthew 12:25-26).
Two things sit inside that one accusation, and both are worth naming plainly: the Pharisees could not deny the miracle itself, and they conceded, against their own argument, that there was an organized kingdom of evil with a single chief behind it.
— Jamieson, Fausset & Brown, Commentary Critical and Explanatory on the Whole Bible (1871), on Matthew 12:24: "Two things are here implied--first, that the bitterest enemies of our Lord were unable to deny the reality of His miracles; and next, that they believed in an organized infernal kingdom of evil, under one chief."
A local idol's name for "flies" becomes, four centuries later, Israel's courtroom word for the ruler of every unclean spirit: the same pattern as Belial, run in reverse: Scripture keeps repurposing the names of false gods as titles for the one true enemy behind them.