A legal system is a collection of man made laws and a structure for enforcement. The mindset of people living in a legal system is "As long as It cannot be proven that I have technically violated one of those laws, I'm ok."
Justice goes further. Our Declaration of Independence said it this way, "we are endowed by our creator with certain inalienable rights." Over and above any human laws there are rights and obligations that individuals have in relation to one another and in relation to the society of which they are a part. Human laws are themselves measured against these higher rights and obligations established by the creator. And so, a law can be unjust. Words like truth, dignity, and equity have meaning in a justice system. The mindset of people living in a justice system is "If my actions are not only legal, but just, then I'm ok."
Today's gospel calls Chrisitians to something more than the legal or the just.
You have heard that it was said, An eye for an eye and a tooth for a tooth. But I say to you, offer no resistance to one who is evil. When someone strikes you on your right cheek, turn the other one to him as well. If anyone wants to go to law with you over your tunic, hand him your cloak as well. Should anyone press you into service for one mile, go with him for two miles. Give to the one who asks of you, and do not turn your back on one who wants to borrow
Jesus knew well that Christians would find themselves living in times and places where there was no justice, and perhaps not even legalism. He knew our human nature and how injustice can turn to anger and violence. And so, he taught them to look beyond this world for justice. True justice will come to every human being but perhaps not in this life.
As Christians we work to make our society one not simply of laws, but of justice. At the same time, we keep our inner peace intact because we know that wether we succeed or fail in our lifetime, in the end there will be true justice.