What you could say, assuming the number of shooting victims per day remained constant, was that people on that street were now 50% less-likely to be killed in a shooting. If you moved enough people onto that street, again assuming a no change in the number of victims, the likely-hood of any individual being shot could be forced into a statistically insignificant number.
The reverse of your hypothetical is basically how high-crime areas come into being. If you have an area where 1 person every day is killed, and half the people leave, you would absolutely say the quality of life in that area declined. Everyone is twice as likely to die.
While per capita is an imperfect number, it's a crazy-good proxy for the thing we worry about – "how likely is crime in this area to affect me?"
