Why do people carry guns?
Many people carry guns for protection against crime. Yet, like George Zimmerman, many of them suddenly start to see criminals (or potential criminals) everywhere. An unarmed black teenager carrying Skittles and a bottle of iced tea suddenly looks like he's up to no good.
Some people own AR-15's in case they need to stop a tyrannical government. But once they buy the gun to protect themselves from tyranny, every action the government takes suddenly becomes another example of tyranny, even when it's something as benign as health care reform or a ban on Big Gulps.
This, more than anything, is why the United States' gun culture is a problem. Whatever your reason for owning a gun, it becomes a self-fulfilling prophecy. If you own a gun to prevent crimes against you, you start to think that everyone is going to commit a crime against you. If you own a gun to protect yourself from government tyranny, you start to think that any government is tyrannical.
And this is what ultimately leads to George Zimmermans. Had Zimmerman never owned a gun, never applied for a concealed handgun license, it's entirely possible that he never would have looked at Trayvon Martin and seen a criminal. He probably wouldn't have stalked him around the neighborhood. And he definitely would not have ended Trayvon Martin's life.
And the self-fulfilling prophecies are what makes gun owners so fearful of losing their guns, their crutches. They need a gun to defend themselves against criminals... and in their mind, everybody's a criminal.