In his Letter from Birmingham Jail, Dr. Martin Luther King uses a perfectly reasonable, even obvious, definition of extremist when he accepts the label. An extremist is one who is extreme in thoughts and actions and can be extremely good or extremely bad. Jesus was extreme in his demand that his followers should love their enemies. Saint Paul, Martin Luther, John Bunyan, Thomas Jefferson, and Abraham Lincoln were all extremists in admirable ways. Dr. King aspires to be an extremist in the service of love, goodness, and truth, as Jesus Christ was, and says:
Perhaps the South, the nation and the world are in dire need of creative extremists.
In his definition of just and unjust laws, King appeals to the concept of natural law, writing:
A just law is a man-made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law.
You will have your own views on which, if any laws are unjust. It you are considering the question in countries around the world, laws imposing the death penalty for homosexuality or blasphemy are obvious candidates. Within the United States, you could look at laws that vary from state to state, since it would be difficult to argue that such legislation is based on natural law. There are various reasons why people obey laws which they think are unjust, including social pressure and fear of punishment.