we could see that some countries is still trying to develop nuclear (weapon). Those countries had been considered as rough countries. They said that nuclear helps them to protect themselves from being attacked by other countries. But, on different views, we could see that nuclear weapons become a serious threat to global security.
A lot of countries want to develop nuclear weapons because they think that they will have an advantage in international affairs. If other countries are afraid of the nuclear weapons, or the threat of nuclear weapons, they can throw their weight around. Some countries think they can push around neighboring countries or trade partners if they have the threat of nuclear weapons.
I think my biggest concern with these countries developing nuclear weapons is the threat of terrorism. Some of these countries are in the middle of or have terrorist training camps. How long before some terrorist group has control of one of these countries nuclear weapon.
Let's take Iran as a specific example. I surely don't want them or any other country developing nuclear weapons. It increases the chance that they will be used at some point. But from their perspective, the United States, a country which has no legal trade with them, just toughened sanctions with the UN, and that invaded countries on both sides of Iran: Afghanistan and Iraq, they feel directly threatened.
They also know that if they were to successfully develop an atomic weapon, then I would guarantee both their national security and gain them an added measure of diplomatic respect around the world. As much as I don't like their position and efforts in this area, I can nevertheless understand it.
Motivation is everything. If a country feels the need to protect itself and is stable enoughto have a trustworthy government built on reasonable and non-aggressive principles, the global community doesn't generally react too dramatically. On the other hand, if a country is already rather bellicose (war-like) and aggressive, its motives don't seem as pure when we learn it's creating a nuclear weapon. Instead of assuming the best, the world presumes the worst and does what it can to stop the process. This is a dramatic weapon; so the reactions and responses, too, are dramatic.
Sure, we can see from a broader perspective that nuclear weapons are a security threat to the international system. But this does not mean that each country will see them as a threat to its own security.
At least according to realists, states act in their own interests, not in the interests of global security. So I would argue that countries think that getting nukes will improve their own security even if that reduces global security.
Are they right to think so? That's less clear, but the example of North Korea at least implies that you can gain benefits from having nukes and threatening other people with them.