1. What happens when "the richest person" doesn't care about killing all of humanity for no worthwhile reason whatsoever.

2. How is this richest person enacting this "join or die" plan going to be enforcing the "die" part of his offer? If he needs them to join then they need the assets of these other people. Meaning they don't yet have the assets for a giant army of doom. Meaning they'd need the help of human mercenaries and assasins. Whom I'm pretty sure would have enough critical thinking to realize that someone planning on wiping out most of humanity wouldn't be the type to let them so much as live just because they did their employer's bidding.

3. How do they plan on getting the world's governments on board? Since, yoy know, nukes still beat Doom robots. And head's of state are not generally know to cave into the demands of wannabe supervillains.

This possibility is even worse than most others sinceeven the other rich people don't want it, meaning their is literally nothing getting in the way of these highly affluent well connected people from going on live TV and saying "hey, I'm being targeted by assassins and need 24/7 CIA protection. This is the person doing it."

Or any other myriad of ways things get leaked. Maybe the lesser rich person offers more than the big rich is paying their mercy and turns the tables with an assassination of their own.

The only way a conspiracy like this works is if everyone is on the same page. When people who actively do not want to kill off humanity are included, it just increases the odds of word getting out.