I'm concerned about the knock-down approach by the FBI. Say I live in a state which allows one to kill home intruders on sight, could I not legally set up an alarm system that sprays some bullets in the direction of the door if it's opened without being disabled? By knocking I can disable it and let them in instead of having a few dead FBI agents on my doorstep.
Of course you couldn't legally set that up, are you insane? On what planet would that be justifiable? Even in Texas, you need to demonstrate a reasonable belief that you were in imminent danger of being robbed or attacked before you are permitted to use deadly force on an intruder in your home.
Ignore the silly mechanism described, and imagine you're sleeping in bed with your wife at 3am, and are a completely law abiding citizen as far as you are concerned. Why would you possibly have any reason to believe that the people violently breaking down your door and and yelling have good intentions?
This isn't hypothetical, this kind of shit has actually happened. Both innocent people, and cops, have died in these sort of situations.