During the Magna carta, why would it be culturally accepted that only under law are people truly free?

During the times when the Magna Carta was signed, people had first-hand experience with what happens when there is no law. Without law, we are at the mercy of anyone who wants to hurt us or take our property. What's to stop them, except physical force. So you fight back, then they come back with more bad people! That's a terrible way to live. Under law, we feel secure that anyone who tried that will be punished by the ruling authorities. Today, through our democratic system, we have the right to choose our rulers because we believe they will do their best to rule well. In King John's time, people had no choice of ruler, but law made even the king responsible to do his duty. That was one of the points of the Magna Carta -- the king is not above the law.