According to the CDC WISQUARS, there are around 300-400 justifiable/legal interbvention gun related deaths a year in America. Perhaps because perps give up when they face a armed person rather than risk being shot.
2010, United States
Legal Intervention Injury Deaths and Rates per 100,000
All Races, Both Sexes, All Ages
ICD-10 Codes: Y35,Y89.0
Number of Deaths 412
Crude Rate 0.13
Age-Adjusted Rate** 0.14