"In those days Israel had no king; everyone did as they saw fit."
The days of Judges were chaos. People killed without mercy for there was no law enforced. The Law existed as given to Moses, but the people abandoned it and did what was wicked in the LORD's sight. This is the result of lawbreaking: chaos. When people start to say to each other "You do want you want and I'll do what I want and we'll all be happy", they simply delude themselves into thinking they have peace.
For when two people both want opposite things, conflict arises and wars break out. People are hurt and many innocents will be laid waste all for the sake of selfish ambition and personal gain. When you don't play by the rules, the game ceases to be fun and enjoyable. Life is meant to be enjoyed, not fought over.
That is why God gave us his law in order that we might know what is good. And this is why he fulfilled the law in Christ in order that we may have peace in him. By the power of the Holy Spirit who enables us to keep the law, he enables us to be at peace and harmony. No longer are we called enemies, but He calls us friends. Now Jesus Christ is King forever so that we might have peace forever! This is why he is called "The Prince of Peace".