I taught from Judges 18 not too long ago. This chapter falls into a section which is often called an appendix to the book. Whether it should be viewed as an appendix or not can be debated. In any case, I think five observations can be made concerning these chapters.
1. This section is illustrative and exemplifies the utter moral depravity and spiritual bankruptcy of Israel. The holy nation bears greater resemblance to the pagan Canaanites than a covenant people related to YHWH.
2. The key text in this section (and perhaps the book) is Judges 17:6: “In those days there was no king in Israel; every man did what was right in his own eyes” (cf. 18:1; 19:1; 21:25).
3. There is a logical and causal connection between a lack of leadership and a lack of morality.
4. This section is dischronologized. The events in chps 17–21 apparently occur early in the Judges period (note the references to the grandsons of both Moses (18:30) and Aaron (20:28) and the reference to the ark located at Bethel (20:27–28).
5. While this section is clearly divided into two parts: (1) 17:1–18:31, (2) 19:1–21:25, there are common links between the two sections.