Ah, doesn't this story keep getting better? No, not really. Apparently all of a sudden, the Israelites start realizing that--especially to the extent that they have butchered an entire tribe of Israel--two wrongs don't make a right. So naturally, they do what anyone would, and decide to see if maybe three wrongs will do the trick.
Since they had made a vow that they would never allow their own daughters to marry one of the Benjamites, they're in a quandary. They want to help the remnant of this tribe rebuild, but they can't do it without breaking their vows. The remaining Benjamites might have women of their own actually, but since there are such a small number of survivors of this genocidal rage we saw in the previous chapter, it seems to me that those women would probably be the daughters and sisters of the surviving men. Or it may be that they truly did destroy every city, and all inhabitants, which would have been all the women, the only survivors being a handful of fleeing Benjamite soldiers. Whatever the particulars of the situation, the Israelites show their moral fiber when they have to choose: On the one hand, they can break their rashly-made and morally-pointless vows, or they can kidnap, murder and pillage in the town of Jabeshgilead, the inhabitants of which are not bound by such a stupid vow and, apparently alone in all Israel in that day, had no blood on their hands. They of course choose the latter; you can't expect them to break their word, can you?
So they kill every man and every non-virgin woman in the city. I have no idea why this was supposedly necessary rather than simply kidnapping the virgins, but I don't understand much that these people do. (You almost imagine, what with the way they've conducted themselves in the previous chapter, that they got together and said, "But what if someone from the town complains?" Think...think...think. "I've got it! We'll just kill everyone, then they can't complain!") They bring the women back to the survivors of Benjamin, but there are only 400 virgins for what I assume is about a thousand soldiers. So, not nearly enough. What to do?
Solution? More kidnapping! (At least they didn't kill anybody else, I guess.) The people assembled, who I assume were not from the region of Shiloh, suggest that during an upcoming festival in Shiloh, the remaining unmarried Benjamites lie in wait for some young women who will come out to dance in the vineyards as part of the festival. If they simply drag these women off and marry them, then it's reasoned that their fathers will not be breaking their vows, since they didn't willingly allow their daughters to go. So they do it.
Summing up, kidnapping, lying, scheming, rape (of female virgins), arson, stealing, and mass murder are all considered to be preferable to breaking one's vow to the Israelites during this time, for "every man did that which was right in his own eyes."
Why am I summing up rather than explaining or excusing? I don't think there's much one can explain, and pretty much no excuse for this behavior. You can't blame it on God, though; you can't blame it on the Mosaic Law. These aren't people acting in the name of God; this is essentially random, bloodthirsty mass insanity.
Why is it here in the Bible? The SAB labels this passage in various places as essentially violent, unjust, misogynistic, and displaying a warped sense of family values. I'd be curious as to whether there is anyone alive today who professes belief in the Bible who disagrees with such an assessment; I certainly don't disagree. On the one hand, the simple fact is that this is supposed to be history. If this event actually happened--and indeed, who would want to make up such an event and put it into their own country's history?--then I think it would be immoral to simply sweep the whole thing under the rug. In the same fashion, Christian believers in the Bible need to deal with this story, and not pretend it doesn't matter. It's from this very nation, this very group of barbaric, bloodthirsty warriors, that the people we consider heroes of the faith arise. King David, the prophet Daniel, John the Baptist, even Jesus himself comes from these people.
The thing to realize from this, and so many other stories in the Bible, is that the Jews are God's chosen people. Not the best people in the world, and despite what you might think from this story, not the worst people either, just the chosen people. As such, they are simply people. What does the Bible tell us about people? "[T]he imagination of man's heart is evil from his youth", God says in Genesis 8. "[T]he heart of the sons of men is full of evil, and madness is in their heart while they live", King Solomon observes in Ecclesiastes 9. Jesus himself tells us in Mark 7 that, "For from within, out of men's hearts, come evil thoughts, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly." And yet, it was this very same Jesus who gave up his life for the salvation of all of mankind. Why did God choose the Jews? Deuteronomy 7:7-8 says, "The LORD did not set his affection on you and choose you because you were more numerous than other peoples, for you were the fewest of all peoples. But it was because the LORD loved you and kept the oath he swore to your forefathers that he brought you out with a mighty hand and redeemed you from the land of slavery, from the power of Pharaoh king of Egypt." Why did Jesus decide to die for sinful people? Romans 5:6-8 tells us, "You see, at just the right time, when we were still powerless, Christ died for the ungodly. Very rarely will anyone die for a righteous man, though for a good man someone might possibly dare to die. But God demonstrates his own love for us in this: While we were still sinners, Christ died for us." It's in the stories of the very worst that humanity has to offer that we see the depth of the love of God: that despite our best efforts to be as evil and reprobate as possible, God still insists on reaching out to us in love. This horrible chapter in Israel's history is not the final chapter; it's only the beginning.