Why did God strike Herod down?

If you read the account linked to in Peter's comment, it sure doesn't sound anything like what you might imagine from the Bible verse alone:

The disease then seized upon his whole body and distracted it by various torments. For he had a slow fever, and the itching of the skin of his whole body was insupportable. He suffered also from continuous pains in his colon, and there were swellings on his feet like those of a person suffering from dropsy, while his abdomen was inflamed and his privy member so putrefied as to produce worms. Besides this he could breathe only in an upright posture, and then only with difficulty, and he had convulsions in all his limbs, so that the diviners said that his diseases were a punishment. But he, although wrestling with such sufferings, nevertheless clung to life and hoped for safety, and devised methods of cure. For instance, crossing over Jordan he used the warm baths at Callirhoë, which flow into the Lake Asphaltites, but are themselves sweet enough to drink.

His physicians here thought that they could warm his whole body again by means of heated oil. But when they had let him down into a tub filled with oil, his eyes became weak and turned up like the eyes of a dead person. But when his attendants raised an outcry, he recovered at the noise; but finally, despairing of a cure, he commanded about fifty drachms to be distributed among the soldiers, and great sums to be given to his generals and friends.

Then returning he came to Jericho, where, being seized with melancholy, he planned to commit an impious deed, as if challenging death itself. For, collecting from every town the most illustrious men of all Judea, he commanded that they be shut up in the so-called hippodrome.

And having summoned Salome, his sister, and her husband, Alexander, he said: 'I know that the Jews will rejoice at my death. But I may be lamented by others and have a splendid funeral if you are willing to perform my commands. When I shall expire surround these men, who are now under guard, as quickly as possible with soldiers, and slay them, in order that all Judea and every house may weep for me even against their will.'

There's more, but the point is he apparently suffered a long, slow, agonizing disease all the while he continued to commit evil acts of cruelty. The entire episode is glossed into the single verse in question.

I've found that a lot of the Bible is missing crucial context like this. I'm finding that I often learn a good deal more by reading about a book of the Bible than by reading the book itself.

The King Herod said to have been struck down by an angel of the Lord must have been Herod Agrippa, since King Herod the Great died decades earlier, in 4 BCE. There are two, quite different accounts of the death of Herod Agrippa.

The most famous account is found in Acts 12:23 -

Acts 12:23 (KJV): And immediately 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.

The first-century Jewish historian, Josephus says that in quite different circumstances, Herod Agrippa suffered such intense stomach pains that he resigned himself to dying, while "all places" were full of mourning and lamentation. Herod scolded his sycophants for flattering him and finally succumbed to this illness with dignity after five days of suffering. Josephus does not mention Herod being struck down or eaten by worms:

Antiquities of the Jews 19.8.2: When he said this, his pain was become violent. Accordingly he was carried into the palace, and the rumor went abroad every where, that he would certainly die in a little time. But the multitude presently sat in sackcloth, with their wives and children, after the law of their country, and besought God for the king's recovery. All places were also full of mourning and lamentation. Now the king rested in a high chamber, and as he saw them below lying prostrate on the ground, he could not himself forbear weeping. And when he had been quite worn out by the pain in his belly for five days, he departed this life, being in the fifty-fourth year of his age, and in the seventh year of his reign ...

Josephus did not always write accurate and unbiased histories. Although here his account is more in accord with the question's observation that God does not interfere and manipulate [but] lets things run their course and has set aside a day for judgement, he may have been influenced by the consistently favourable view the Jews apparently had of Herod Agrippa. Christians believe that Acts is the more accurate history and that Herod was struck down for allowing the people to acclaim him as a god. There can only be conjecture as to why Herod was struck down so immediately.

Interestingly, Josephus says that Agrippa's grandfather, Herod the Great died an excruciating death, with worms coming out of his penis, raising the possibility that the author of Acts simply confused the two kings Herod:

Antiquities 17:6:5: But now Herod's distemper greatly increased upon him after a severe manner, and this by God's judgment upon him for his sins; for a fire glowed in him slowly, which did not so much appear to the touch outwardly, as it augmented his pains inwardly; for it brought upon him a vehement appetite to eating, which he could not avoid to supply with one sort of food or other. His entrails were also ex-ulcerated, and the chief violence of his pain lay on his colon; an aqueous and transparent liquor also had settled itself about his feet, and a like matter afflicted him at the bottom of his belly. Nay, further, his privy-member was putrefied, and produced worms ...

The Bible does not anywhere indicate that the ‘most’ evil people are killed directly by God. God rather often, according to the purposes of his will, postpones the eternal judgment for sin a long while. Even the very worst of criminals may live long and prosperous lives and we must not expect full justice in this world. The injustice in the world is part of the judgment for Adam’s sin. It is only in certain situation that God hastens judgment directly to bring attention to certain things that people might otherwise think God is not that concerned about, or for other mysterious reasons according to Him ‘who works out everything to the good to those that love him’. (Rom 8:28). Things that everybody knows are evil God often overlooks as know special point needs to be made.

It might be asked ‘Why does not God kill more people, or even all people, like in the days of Noah? But God’s postponement of justice is not the absence of justice, which is what hell is for. It is only his mercy and intentions of grace in Christ (signified by the rainbow) that keeps God from killing more swiftly each new generation. Death itself is in a sense God killing mankind.

As it turns out on this earth, it sometimes almost seems ‘random’ who God directly causes to die and he alone knows his sovereign purposes. When God does miraculously step forward to execute sudden judgment he seems to be sending an important message in line with his purposes at that time of history.

For example in the Old Testament he killed Uzzah for Touching the Ark while trying to prevent it from tipping over (2 Samuel 6:6-7) which makes it seem singled out to be made an example. Meanwhile, while others did more wicked things God may have had no direct interest to intervene according to his own purpose.

In most cases however, it seems God takes direct actions when the people involved are committing public, almost symbolic sins, to put fear in the hearts of those observing and to therefore prevent similar sins by others. For example, Ananias and Sapphira lying to the Holy Spirit in Acts 5:1-10. In the case of Herod he was busy persecuting the newly founded church and allowing others to worship him as a god, all the while pretending to be a Jew and a king of Jews. This was simply something, according to God's direct purposes for his church, that God wanted to publicly show as not acceptable.

Furthermore, just because the Bible does not mention God's involvement in killing other wicked men, history is filled with crazy dictators and especially vile public figures that have all died sudden and horrific deaths at young ages or otherwise killed themselves such as Judas, Nero, Hitler, etc.

I am not aware of God directly killing (or other dramatic judgement) a person for private sins — it seems always related to 'scandalous public affairs' that if gone unpunished in the eyes of men might affront God's purposes among certain groups at certain times.