[archer51]

The situation you described is why carrying a non-lethal alternative is a good idea. Once he drops the weapon deadly force is going to be difficult to defend. Not saying impossible, but difficult. If he possessed no weapon at all, drawing your weapon is going to be difficult to justify as well. The use of pepper spray instead of lethal force will show you did what you had to, to protect yourself and dad without using a gun.

On a side note, you dad, being partially disabled might stand a better chance defending the use of lethal force in the scenario above.