As we have all come to understand WMD have nothing to do with threats of national existance. That is not their purpose. Any country in the 21st Century would have little rationale to use WMD of any kind to insure it's survival because that would not be guaranteed anyway.
No the utility of WMD is the threat of someone else's destruction, not the insuring of one's own. That is a clearly different motive. Think of it more along the lines of multilateral MAD (mutually assured destruction). T
The remaining question then is what political gain is there from the threat? In Iraq's case, any ability to use WMD is a lever by which any other military or political aggression can be accomplished. A nuclear Iraq can invade Kuwait or Turkey or Saudi Arabia precisely because it has demonstrated to use whatever it wants whenever it wants to. The West would not block a nuclear Iraq from invading another country with non nuclear means.
In the case of North Korea the rationale is somewhat more murky for different reasons. The fear from North Korea is the fear theat they will export the technology to whomever pays for it. They have already demonstrated an ability to be the terrorist worlds ballistic missile supplier.
The case of Iran is yet another case. Their senior clerics have publically stated that the day they acquire a nuclear weapon is the last of existance for Israel. They have made clear their intention to use it. And the rest of the world should not underestimate their willingness to do so. A country that would send a million of its own unarmed teenagers against Iraqi tanks and chemical weapons can be taken at face value.
The case for Israel is far different. For one thing, unlike Iraq that signed the NPT and then broke it, Israel has an opaque policy. Not a yes, not a no. There is a great deal of flexibility in such a policy. Since they have not signed the NPT they are not bound by it and clearly no country should be expected to be compelled to sign any treaty. Next what is different is that because of this opacity Israel has never made the claim that they would use WMD or under what circumstances they might. There is a reasonable body of evidence to indicate that as early as pre 67 Israel has an operational nuclear capability. Since then, if that is true they have had ample opportunity to consider their use but did not.
Addtionally one must consider that this decision is extremely complex and there are several different scenarios with different tactical decisions attached to each. There simply is no single 'launch no launch' switch. There are probably dozens as there are probably more than 5 or 6 different political and military scenarios that drive them.
The most troubling problem though doesn't have to do with WMD. It has to do with the defence from WMD. One plausible situation is a launch from Iraq. An optimal case is that Israel has 3-4 minutes advanced time from detectionto respond. By that figure alone clearly the Israelis will be in a situation to shoot down missiles that are over the West Bank or Jordan. Politically, what are the ramifications for them to shoot down a missile that rains Vx or Sarin or phosgene on the West Bank? Damned if they do, damned if they don't.