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 form 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.