So called "castle doctrine laws" have been in the news as a result of the shooting death of Trayvon Martin, in Florida, and the subsequent trial of George Zimmerman, a neighborhood watch member who says he was defending himself. The castle doctrine extends a legal principle that exists in one's house — a citizen has no duty to retreat when threatened with violence before using violence in self-defense — to many other situations. Florida was the first to pass such a law, in 2005, and more than 20 other states have since followed its lead.

In...