The Memetic Virus
The Cause -
A lab working on cutting edge psychology makes a serendipitous breakthrough - the keys to the human mind. With the right images, words and triggers, you can program anyone with permanent instructions that they'll then happily follow.
Unfortunately one of the researchers is significantly lacking in moral integrity. He uses these techniques to first take control of everyone else in the lab, ensuring no one can use the same techniques against him and eliminating any rivals. He then creates a literally viral video, that instructs people to take his orders, and to spread the video enthusiastically to everyone they know, and to help him translate it to different languages.
Once he has a significant portion of humanity, he moves to more open measures. The researchers translate the control video into different languages, and with different cultural context. It's now played around the world once an hour on every television station, and people without TVs are rounded up and forced to watch (at gunpoint if necessary), as are those who live extremely isolated lives. Less than a week after the discovery, he controls over 99% of humanity.
Thus, the Overlord is born.
Why they died -
While it works on almost everyone, it turns out that a small fraction of humans - perhaps a hundreth of a percent - are immune to the effects. Their brains just aren't hardwired correctly for it to work on them.
It's impossible to tell exactly what happened, but the best theory is that some of the immunes managed to infiltrate the Overlord's palace and assassinate him. Unfortunately for everyone, the Overlord had put some failsafes into place:
- Shortly after his death, his new palace exploded. A significant nuclear weapon was used, and if anyone did live after assassinating him, it wasn't for long.
- The Overlord had decided that if he died, he was taking everyone else with him. Buried in everyone's instruction set were orders that if they didn't get new instructions within 48 hours, they were to kill themselves.
So two days after the explosion, almost all of humanity stopped what they were doing, walked to the nearest body of water, and drowned themselves.
Why Bob Lives -
While the Overlord made a significant effort to hunt down people in remote locales, Bob's island is marked as uninhabited and no one knows he's there.
However, it's reasonable to assume that at some point Bob would get exposed to this video. If it wasn't running normally, he might find it accidentally while investigating what happened. There's a few possible reasons he might live after that:
- He's one of those immunes. This is pretty unlikely though.
- He's deaf, and couldn't get the full instruction set.
- There's a loophole in the suicide order. It tells people to kill themselves exactly 48 hours after the message was first broadcast. Since it's been almost a week and Bob can't time travel back 5 days, the order is impossible for him to fulfill, so he ignores it.
Edit: it should be noted that since the Overlord didn't have the resources to directly command 7 billion people, the instructions largely just made sure that people would follow his orders. Since he's no longer around any more and hasn't given Bob any specific orders, Bob is mostly unaffected.