Why do some people want everyone to have a gun, but also don't want every country to have nukes?

Short answer: because an atomic bomb can destroy a city, killing hundreds of thousands in a millisecond. Not only that but you can't really control the situation with an atomic bomb. They can fit in brief cases now, which is terrifying. Guns on the other hand would need impossible amounts of ammo and for people to do nothing to do the same amount of damage. Generally, that doesn't happen. If everyone has a gun, everyone is safe because everyone could defend themselves. If everyone has a nuke, everyone is dead.

Incorrect.

No one wants "everyone" to have a gun. We don't want criminals and folks who are mentally dangerous to have them.

Kind of like we don't want Iran to have nukes either.

There's a reason nobody gives a damn if France has nukes, just like nobody should give a damn that I own weapons. Because I'm not a threat to others, just like France isn't.