Why did God kill a non-Jewish ruler?
Acts 12:21-22
21 And upon a day appointed, Herod being arrayed in kingly apparel, sat in the judgment seat and made an oration to them. 22 And the people made acclamation, saying: It is the voice of a god, and not of a man. 23 And forthwith an angel of the Lord struck him, because he had not given the honour to God: and, being eaten up by worms, he gave up the ghost.
There were plenty of other people involved in the deaths of the Apostles, yet God did not execute vengeance then. Consider the beheading of John the Baptizer.
http://www.endtime.org/library/articles/leavenofherod.html
To the Herodian characteristics of his predecessors, Herod Agrippa added the leaven of vanity, and love of popularity along social as well as political lines. We are told that he "stretched forth his hands to vex certain of the church. And he killed James . . . with the sword. And because he saw it pleased the Jews, he proceeded . . . to take Peter also." But Peter escaped from prison, and Herod ordered his keepers to be put to death. Then "upon a set day Herod, arrayed in royal apparel, sat upon his throne, and made an oration," and the people said, "It is the voice of a god, and not of a man." Self-deification quickly brought its own punishment, however, for the narrative goes on to say that "the angel of the Lord smote him, because he gave not God the glory: and he was eaten of worms, and gave up the ghost."