This verse marks the first instance of the key refrain in the closing chapters of the book of Judges (Judges 17:1 – 21:25). It gives us the interpretive key for these chapters.
Is the author saying that the lawlessness was because there was no king in Israel to hold the nation to the Lord’s standard of righteousness? At first glance it appears that way, but this need not be the case. The author, writing this much later, possibly during the time of Manasseh, had seen how immoral Israel could be under a wicked king; in fact, it was the monarchy that was chiefly responsible for Israel’s apostasy that eventually resulted in the downfall of both the northern and southern kingdoms. The difference between the period of the governors and the monarchy/ies is not the presence or absence of idolatry or evil. Rather, it is the source from which the evil springs. During the monarchy, kings led the way in abominable acts; in premonarchic times the people did it on their own.
1 So the author of Judges observes that in the dark days of the judges, even without a king to lead them astray, the people behaved wickedly all on their own. A comparison can be made with a king like Jeroboam. Like Micah, he set up a house of worship at a place of his own choosing (see 1 Kings 18:29–31), built images for worshipping God, and installed his own priest at the shrine (see 1 Kings 13:33). This was a mixing of true and false religion together. The judges behaved just like later kings, who also were denounced for not doing what was right in the Lord’s eyes (Solomon (1 Kings 14:8), Jehu (2 Kings 10:30), and Ahaz (2 Kings 16:2)). Israel needed a ruler, yes, but the right ruler. They did have that in the Lord. But they ignored him. They want their own sort of religion.2
6 In those days there was no king in Israel. Everyone did what was right in his own eyes.