I'm inclined to think that "warning shots" are merely cover for missing the target initially.
Burglar: Gimme all your money!
Victim: Bang!
Burglar: You missed!
Victim: That was a warning shot!
Burglar: I don't feel warned.
Victim: Bang!
Burglar:...croak...
time passes. 911 is called.
Cops:He's dead, Jim.
Victim: I fired a warning shot, and he kept coming. So I fired again and stopped him. I was in fear of my own life.
Cops: Sounds right to me. Let's go get some ice cream!