Herod the Tetrarch
Historically, Herod was "tetrarch" of Galilee and Perea from 4 BCE to 39 CE. We know what you history buffs are thinking, and no, this isn't Herod the Great. That's the tetrarch's dad, who was "King" over a large swath of Palestine from 40 to 4 BCE. Dad gets some screentime in 1:5, but it's the tetrarch in action everywhere else Herod is mentioned (3:19-20; 9:7-9; 13:31-33; 23:6-12).
Being tetrarch wasn't as prestigious as being king, and Herod knew it. In 39, Herod was actually fired from his job as tetrarch when he sailed to Rome in order to request a promotion to king. The Romans basically told him to think again, buddy. Oops.
The Villain of All Villains
Herod does not come off very good in Luke's gospel. He is a villain pure and simple—no hint of grey, no redeeming qualities, a veritable Sauron. It's worth mentioning, since the situation is different in Mark's gospel, where Herod is portrayed as a somewhat fearful and conflicted ruler who's easily manipulated by others (check out Mark 6:14-29).
In Luke, Herod's criminal record is quite thick. He arrests and beheads John the Baptist (3:20; 9:9) for being an outspoken critic of the tetrarch. And he directs this same animosity toward Jesus, whom he seeks "to kill" (13:31). Finally, when he meets Jesus in Jerusalem, he tops it all off by mocking Jesus, dressing him up in royal attire and returning him to Pilate as one big joke (23:6-12).
Knowing that the Devil is in charge of appointing rulers in the Roman world (see 4:5-7), it's no wonder Herod's such a jerk.