At some point the possession of weapons rises to the level of a threat against other people's lives and property. Clearly, at one extreme, a pocket knife does not qualify on this count, since there are numerous legitimate reasons why someone might have a pocket knife (among them being self-defense). On the other hand, weapons of mass destruction by their nature cannot be used for legitimate self-defense (since a large amount of collateral damage will result), and so the possession of such weapons constitutes a threat of aggression. By analogy, if I point a loaded gun at your head and promise not to shoot, then even though I have not "actually" invaded your person or property, my actions constitute a threat irrespective of my words, and you would be entitled to make me stop pointing the gun at you. I might protest and say that my intent is not to shoot but to live out my childhood dream of being a cowboy (or some other such strange reason), but no reasonable person would conclude that going so far as to point a gun at you would cause you to feel anything but fear for your life/property. So it is with nukes.
I suppose it could be argued that a nuclear bomb located in a remote area could possibly have legitimate, non-aggressive uses (e.g., an offshore oil drilling company might want to detonate a nuke to blast a hole in the ocean floor), but certainly having one in a populated area should not be allowed.