Do you know why that didn't happen in Richmond CA when they did it? Specifically reports show a 77% drop in homicides (from another comment). Or what were the "cobras" that sprung up there?
I suspect the odds of unexpected outcomes would be low here though because the program itself is small (only 10 people). And there are inherent downsides to engaging in behavior that make others think you're likely to shoot someone, so it's not something you do on a whim.
If you believe in natural selection and survival of the fittest, you’re increasing the fitness of the potential murderers over the peaceful people.