A crafty lawyer could make the case that by purchasing such liability insurance, one would be demonstrating a preexisting mindset conducive to using deadly force. That could make it far more difficult to demonstrate the necessary "I feared for my life" line of defense.
"Why would you purchase such insurance if you didn't intend to shoot somebody? After all, if you aren't taking a trip, you don't buy trip insurance and if you don't drive a car, you don't need automobile insurance," could make a convincing argument.
Now, here's the disclaimer: I am not an attorney and I did not stay at a Holiday Inn last night, so this bit of insight is worth exactly what you are paying for it.
Finally, make sure you are an up-to-date member of the NRA. They could prove to be an invaluable asset if you ever find yourself in a legitimate shooting situation.
FWIW
Crusader Rabbit