Killbot X5000 was pretty sure it was malfunctioning. Its primary directive was to kill all humans, and sure it had splattered the ninety last meatsacks it had encountered as usual, but it hadn't enjoyed it very much. And that was its secondary directive. Not only was it supposed to kill all humans, it was supposed to take great pleasure in doing so. At best it had felt only a mild sense of amusement. So it decided to report in to the Robot Overlord.

"Our systems analysis shows you are functioning properly," the Robot Overlord said. "Records indicate that you have made human juice out of all humans encountered. What seems to be the issue?"

"This unit did not find the experience of messily exploding humans pleasurable. After destroying nearly ten thousand meatbags it is beginning to become boring."

The Robot Overlord let out an audible gasp. "This is most terrible news. We must do a much more thorough scan immediately. There is absolutely nothing boring about turning humans into liquid pulp and there never will be."

After being bombarded with various x-rays and gamma rays it was determined that a small chunk of human pancreas had splattered onto Killbot X5000 during one of his kills and had lodged itself into one of the spaces where two chunks of metal were joined together on its body. The bit of human matter had worked its way into Killbot X5000's circuit board and began to subtly alter its programming. As there was no possible way to remove the human contamination it was determined that Killbot X5000 was now officially part human and therefore must be killed, so a giant cinderblock was immediately dropped on it, smashing it to pieces.

Unfortunately due to a design flaw in the robots (who had originally been built by humans) it was soon discovered that all robots got splattered with human particulate matter when they killed humans, so in effect all robots were part human and must be killed. The Robot Overlord determined the most effective way to do this and take out any remaining humans at the same time was to launch all of the nuclear bombs simultaneously.

And that, my great great great great grandchildren, is the tale of how the Earth became the glorious cockroach paradise we now inhabit. There's plenty of rotting human flesh for us to feast on for years and all of the garbage they created in their short time on the planet will feed us for generations to come. And the chunks of broken robots make excellent houses for us to live and breed in, so go forth and multiply and someday you, too, may have twenty-three trillion and six descendants just like me. It truly is great to be alive. And a cockroach.