Whilst in theory countries should have the right to develop nuclear weapons, I think we can all appreciate how problematic this becomes in practice.
If countries are sitting around with a vast nuclear arsenal that is ready to go, we can only assume that they will use them at the first provocation, which would no doubt lead to nuclear retaliation and before you know it, an all out nuclear war.
War is, sadly, a regrettable inevitability in the modern world (and also historically). The damage caused by military attacks is awful as it is, it would not bear thinking about how much worse it could and would be if nuclear weapons become the norm in all warfare.
Check out this video to see what nuclear bombs would do if used today: