This summer I have landed the role of King Herod in MIT MTG's production of Jesus Christ Superstar. I'm pretty excited about this since I've wanted to do this role for a long time. I'm working my way through all the biggest d-bags in theater, and Herod is high on that list. Plus, not getting this role the last time this group did the show (about a million years ago) is pretty much why I started taking voice lessons, so it's all come full circle.
Herod as depicted in the show and in the bible (Matthew, Mark and Luke) is Herod Antipas, the son of Herod the Great. Herod the Great was a strong and totally batshit insane Jewish king, who was known for things such as expanding and renovating the Temple into opulence and also having his own family executed. Herod the Great is the Herod who orders the Massacre of the Innocents, described in Matthew chapter 2, in order to get rid of the one who was born King of the Jews and might thus take his throne.
See, in addition to being insane, the Heroditans had a problem. Their legitimacy was pretty shaky--for starters, Herod the Great was a convert to Judaism. Second, his line was installed as tetrarchs in Judea, basically for having correctly picked the winning side in the Octavian vs Antony throwdown in Rome. Judea's provinces were vassal states of the Roman Empire during this time, and were ruled both by a Roman governor and a local tetrarch. Appointing a ruler from the local population who was loyal to Rome was a common strategy for the Romans, allowing each region the illusion of self-governance. But for the Jews, legitimacy was in question due to Herod's problematic religious background, and further because he had a tendency for ostentatiousness, which orthodox Jews would have frowned on.
For Herod Antipas it would have been even worse. His dad was a convert. His mom was a Samaritan. His crazy dad killed all the elder heirs, and then divided up judea between his surviving sons in his will. Herod went to Rome to try to sleaze his way into being sole heir, but ended up only with Galilee and Peraea. Herod built a new capital, Tiberius, in honor of the emperor. Of course, he build it on a Jewish graveyard, and no Jews would enter the city. He divorced his first wife to marry his brother's hot widow, which irritated the Jewish priesthood. Josephus, the Jewish historian, writes of how terrified Herod was of John the Baptist's popular support, and that he killed him out of fear. While Josephus is silent on Jesus, the Gospels indicate Herod basically feared that Jesus was John the Baptist somehow back from the dead, and viewed him as serious a threat to the throne.
Herod and Pilate didn't get along, and some historians think this is because Pilate had put to death a number of rebellious Galileans without consulting him. This act upset Herod, who considered that his jurisdiction. So when, in the bible/show Pilate gets a hold of Jesus, but doesn't want to be responsible for executing a man he viewed as basically harmless, he throws him to Herod to let Herod try him (Jesus being from Galilee).
I also think in the context of the Pharisee plot as presented in Jesus Christ Superstar, this was a move by pilate to shift Jesus' crime from that of treason against Rome, punishable by crucifixion, to blasphemy against the Jewish God, a crime under Jewish but not Roman law. If Herod determines that Jesus is guilty of blasphemy, he'll be punished by the laws of Galilee--I don't know what this would have been, since of course the Bible advocates stoning for acts like claiming you're the son of god, but the show implies non-fatal (corporal punishment or exile, i guess).
My take on Herod's Song, which presents King Herod mocking Jesus before sending him back to Pilate, is that Herod intended to find Jesus guilty of this crime, accepting Pilate's piece offering, and letting Jesus live, basically. Well, he'd probably actually acquit Jesus if he performed miracles-as-parlor-tricks like he demands, but Jesus clearly won't do that. What Herod is looking for is for Jesus to grovel and/or burst into tears, beg for Herod's forgiveness and accept Herod as the King of the Jews. Remember Herod almost certainly saw Jesus, like John the Baptist, as a threat during the height of his ministry, so the chance to humiliate Jesus before the court and prove that he's just a pathetic street preacher with no claim to the throne.
Of course, Jesus neither begs for forgiveness nor puts on a magic show. Herod gets pissed that Jesus isn't palying hsi part, so, unfortunetly for Pilate, throws him back to the Roman court, essentially declaring that Jesus's crime is temporal and against Rome, inciting rebellion and laying claim to the throne given to Herod by the Emperor. This sticks Pilate with having to put Jesus to death to appease the locals.
But apparently Pilate and Herod became the bestest of friends after the trial of Jesus, so there's a happy ending!