And when it was day, certain of the Jews banded together, and bound themselves under a curse, saying that they would neither eat nor drink till they had killed Paul.
Note 1 at Ac 23:12: This is a direct fulfillment of the prophecy of Jesus, where He said, "They shall put you out of the synagogues: yea, the time cometh, that whosoever killeth you will think that he doeth God service" (Joh 16:2). The curse taken by this band of Jews would invoke a curse from God if they failed in their mission. That's how sure they were that they were doing God a service by killing Paul. They felt completely justified in requesting the chief priests' help with the plot, and the chief priests showed no reservations about participating.
How blind can people get! They were enraged at Paul for supposedly proclaiming salvation without holiness (see note 13 at Ac 22:22), yet they were lying and seeking to commit murder. Just as Jesus said, those seeking to cast a speck out of another's eye should remove the beams from their own eyes first (Mt 7:3-5). These were the blind leading the blind (Mt 15:14).