/sigh... we were doing so well there, and then you lapsed back into terrorism.
It's simple, really. Only an objective threat matters. Because you "feel" threatened doesn't matter one whit, if you're not actually threatened. If you are actually threatened, you are being aggressed against, whether you "feel" threatened or not.
I could care less about the fissionable material. It's the fact that the bomb is primed to explode that is the problem. You are more than welcome to draw first and shoot - always remembering that if it turns out he was reaching for his keys to get into his apartment, you may well find yourself in some hot water.
And no, I am not saying you cannot "defend" your family, but that if you choose to stop him from firing by instead shooting him, you are going to need to be responsible for your actions. I notice that you have not mentioned the one course of action which would handily stop a shooter just out for some target practice - as he has informed everyone of - from shooting until your family is clear: Stepping in front of the rifle. That would cause any responsible shooter to immediately safe his weapon, if not completely put it down. Are you afraid to take risks with your own safety to ensure that of your family's?
Again, only an objective threat matters. And an armed nuclear bomb is an objective threat to all within the range of the device. A holstered weapon is not an objective threat to anyone, much less someone who has a nuke wired to his vitals.
What makes you think it would be on their spare time? If there is a bounty, there will be bounty hunters. And I have already explained that (and why) I believe such trades would be significantly rarer in an AnCap society, and we're powerless to stop them now, regardless, so we're pissing into a fan here anyway.
1) You expect me to act rational? I just told you I'm prepared to carry a nuke. I want to get the robber back. That's what matters to me. Others are collateral, and not my concern. I might even conceal my weapon if showing it means I won't get lunch.
2) No threat in unholstering and taking the safety off my gun, right? Not even aiming it in your general direction. I'm not aiming at you, you see. Honest. When does it go from a subjective threat to an objective one? When the bullet leaves the gun? Or just before that, when I actually take aim at you? At which time you're allowed to aim back, right?
3) Yet I can carry an AK47 or similar on the streets, and that's not a threat, right? Safety on, not aiming at anyone. How long does it take me to aim and fire that gun? Am I threatening everyone within distance?
Someone determined to shoot while there are people between him and the target is clearly not reasonable. If he can shoot at targets on the other side of the park, ignoring people that might be in the way, I can clearly do something similar and empty my gun into the ground, ignoring someone just out for some target practice.
4) A rapid fire gun in a quick draw holster is a threat? Or not?
5) You didn't answer what you expected the end result to be if we were to give nukes to Al-quaeda and similar organizations. And how many mushroom clouds do you think will be necessary before people assembles some bounty hunters? And let's hope the seller doesn't sell in bulk. You know that OBL considered it a religious duty to acquire nuclear or biological weapons? Do you think he was the only one? And we're not powerless to stop them now. It's just hard. Only states have nukes now, and they're very regulated. North Korea is the only one that has managed to build them lately, and they're torturing their population to be able to do it.