My understanding is that in most criminal codes in the USA, if not all, defense of property does not justify lethal force. You want to be sure you are in fear of your life (or those of your loved ones).
It's a matter of degree. Most states have adopted the Castle Doctrine from English common law. A person may use deadly force to defend their home, car, tent etc. from an intruder.
http://en.wikipedia.org/wiki/Castle_doctrine