Symbolism, Imagery, Allegory
Okay, this one seems easy enough, right? This stuff represents the atomic bomb. Ice-nine is a polymorph of plain old ice and was developed to make war easier to win—just like the atomic bomb. John just happens to be studying the atomic bomb when he learns of the substance. It even destroys the world in a way eerily similar to atomic winter.
In fact, the connections are so obvious that maybe we can just kick off a little early today and play some Halo.
A is for Atomic
Or… we could dig a little deeper. Here's what Vonnegut had to say on the subject during an interview:
But for me it was terrible, after having believed so much in technology and having drawn so many pictures of dream automobiles and dream airplanes and dream human dwellings, to see the actual use of this technology in destroying [Dresden] and killing 135,000 people and then to see the even more sophisticated technology in the use of nuclear weapons on Japan. I was sickened by this use of the technology that I had had such great hopes for. (Source)
In a way, ice-nine represents more than the atomic bomb. It represents an unchecked use of technology where humanity is harmed rather than helped. When Vonnegut was growing up, technology for him represented ways to help humanity. And while the occasional plane crash or electrical fire might kill someone, neither planes nor electricity were designed for that purpose. The atomic bomb has just one purpose in Vonnegut's eyes: to kill.
D Is for either Deterrent or Deadly
Some people argue that the proliferation of nuclear weapons acts as a deterrent to war, just like they might argue that carrying guns openly deters violence.
In Cat's Cradle, that argument is bunk. In Chapter 22, Dr. Breed claims over and over that "there is no such thing as ice-nine" when it does exist (22.9). After "Papa" Monzano consumes the stuff, John and the Hoenikker children attempt to clean up the mess, but Papa's corpse accidentally tumbles into the sea due to a freak accident involving a crashed airplane.
The point reads clearly: ice-nine's very existence is a threat, and the most well-intended promises and actions of men don't amount to much. Why? Because accidents happen. The jump to nuclear weapons isn't a long one to make, but just to back us up, here's Vonnegut one last time from the same interview:
As for nuclear weapons, I can't imagine why anyone wants them. I don't want my country to have them. I don't want anybody to have them. […] they threaten the entire planet. (Source)