Theory/Headcanon:
We’ve seen Zib before.
Not in the comics or the movie or the actual show, but in the pilot episode of Invader Zim.
And here he is. Pilot Dib IS Zib.
Now, before we get started, here’s the full pilot episode of Invader Zim on YouTube, just in case you haven’t seen it or need context:
If the Zimvoid is actually the Pilot Universe and Zib is actually Pilot Dib, it could explain just how Zib was able to kill Zim, along with why he went off the deep end afterwards.
One of the first things that you’ll probably notice about the Pilot is how Zim and Dib are slightly different.
Starting with Zim, ignoring the different voice, he seems friendlier than his canon counterpart. He casually talks to several of his classmates and compliments Dib for his food canon, which the canon Zims (plural because the Comics and Enter The Florpus are separate timelines) would never do. Pilot Zim is still Zim, but he’s slightly friendlier than normal.
Meanwhile, in sharp contrast to Pilot Zim being friendlier than normal, Pilot Dib is a lot more unhinged than either of his canon counterparts. He’s still Dib at his core, but he’s definitely more unstable than normal.
Combine those two differences, and you start to see how Pilot Dib could kill his Zim.
Zim has a moral code. He views Dib as a worthy opponent and often goes easy on him to give him a fighting chance. Meanwhile, Dib just wants Zim dead with no care as to what he has to do to achieve it.
So mix a friendlier Zim, who might go easier on Dib than what’s normal for Zims or possibly even not treat Dib as a real enemy at all, but instead a frenemy of some sort, with a Dib that will take any opportunity to kill him because that’s a fundamental part of all Dibs, and guess who comes out on top.
Pilot Zim most likely viewed Dib as a rival or frenemy, while Pilot Dib viewed Zim as something to be destroyed. Pilot Zim was also probably a lot more careless when dealing with Dib, as he didn’t view him as the same sort of threat that other Zims view their Dibs as.
Pilot Dib is equally as powerful as any other Dib, probably even more powerful considering his instability, but his Zim didn’t treat him that way. Pilot Zim didn’t treat his Dib like the threat he actually was. In the Pilot Universe, the Zim is weaker than normal and the Dib is stronger than normal.
But there’s also this scene in particular from the Pilot:
Dib tries to prove that Zim is an alien, gets called crazy and looses it, setting his food launcher to the highest setting and aiming it directly at Zim, fully intending to kill him while not caring about the dozens of his own classmates that are stuck to Zim’s mech suit.
This results in Dib’s plan literally blowing up in his face, destroying the entire cafeteria.
Now I’m not saying that this scene is foreshadowing Zib’s destruction of his own universe, where he kills Zim and finally gets evidence, only for no one to believe him, so he looses it, takes over the Earth and tries to destroy the entire Irken Armada with a massive superweapon, wanting to kill every last irken while not caring about Earth or anyone on it, which blows up in his face and destroys his entire universe, except that’s exactly what I’m saying because this scene parallels it perfectly.
This scene from the Pilot is just Zib’s story on a smaller scale.
There’s also the fact that Pilot Dib is the first ever Dib to ever appear in a meta perspective. He’s the first Dib. Dib #1. Which directly parallels Zib’s title of Number 1 in the Zimvoid.
