Hoekstra recently decided to analyze national crime statistics to see what happens in states that pass stand your ground laws. He found the laws are having a measurable effect on the homicide rate.

"Our study finds that, that homicides go up by 7 to 9 percent in states that pass the laws, relative to states that didn't pass the laws over the same time period," he says.

As to whether the laws reduce crime — by creating a deterrence for criminals — he says, "we find no evidence of any deterrence effect over that same time period."

...

based on the available data, it appears that crafters of these laws sought to give good guys more latitude to defend themselves against bad guys. But what Hoekstra's data suggest is that in real-life conflicts, both sides think of the other guy as the bad guy. Both believe the law gives them the right to shoot.

http://www.npr.org/2013/01/02/167984117/-stand-your-ground-linked-to-increase-in-homicide