Because even fatal bullet strikes are rarely instantly fatal. Many times people fire once, achieve what will eventually be a fatal hit and are then killed by the perp.
If her son had been hit four times, or three times, or two times...or, even once...she would still be upset. This is because her son is dead. Whether it was one hit or a dozen is immaterial to her. She has to complain about something.
During the event:
1. What caliber was the firearm used to stop the threat?
2. At what point was the threat, or the potential of continued threat neutralized?