People across the nation are outraged over Trayvon Martin's killing at the hands of vigilante George Zimmerman.

The teenage boy was walking home from a Florida 7-Eleven store with a bag of Skittles and a can of iced tea when Zimmerman stalked the boy and shot and killed him. Even more shocking, Zimmerman wasn't arrested for the crime because he claimed it was in self-defense.

Self-defense, even after he followed Trayvon, carried a gun, was much larger than the boy and has had previous run-ins with the law.

Because Zimmerman is white and Trayvon is black, many have focused on racism as the issue with both Zimmerman and law enforcement, but also at issue is Florida's "Stand Your Ground" law. Montana has a similar law, referred to as the "Castle Doctrine."

That law, passed in 2009, makes it legal for people in Montana to brandish weapons and to use them -- including with deadly force -- if they feel threatened. There is no obligation to try to summon police or to attempt to flee the situation. Furthermore, the burden is on law enforcement to prove that the act wasn't in self-defense.

Shortly after it passed, the ramifications of the law became apparent.  In one instance a Billings Wal-Mart employee shot a co-worker in the face with a .25-caliber semiautomatic Beretta handgun but was released from custody because he said he did so in self-defense. At the time, Yellowstone County Attorney Dennis Paxinos said that prior to the passage of the Castle Doctrine, authorities would have had probable cause to arrest the man for assault with a weapon. He was never charged with a crime.

Now Trayvon's killing is bringing Castle Doctrine and Stand Your Ground laws under renewed scrutiny. At least 21 states in addition to Montana have such laws.

No one should be charged with a crime in a life-or-death situation of self-defense, but neither should these laws be used to allow murderers to remain free.