If you're gonna fire a warning shot, you might as well just yell, "Hey I'm over here, SHOOT ME!!!" Also, it displays what might be interpreted as an unwillingness to kill, which the BG can only use as leverage against you. No matter how willing you are to kill, a warning shot is an all around bad idea.
Another note on this. discharging a firearm is no small thing. When you pull the trigger, it must be to do something. Firing a warning shot can be a reckless endangerment of those around you. The typical warning shot is into the air, and bullets do in fact, come down. Even if you shoot into the ground, warning shots are still a bad idea.