In society, we live according to some rather basic rules (Note; this is meant for a much more simplified network of laws as opposed to the chaotic library of whimsical legislation we have currently). These rules exist so that members of society may live, work, and exchange without having to be continuously paranoid of each other.
Those who have demonstrated a consistent inability to live and function according to these rules should be removed from society with minimal cost (note: "It costs more to kill a person than it does to throw them in jail their entire life" is an artifact of our legal system that can be easily rectified). That means exile or execution. If you can't find a means of exile - execution suffices quite nicely.