But if he had an "aggressive form of cancer" .... well let's move on.
The link you provided claims that people are dropping like flies "in the prime of their life."
I will give you some reasons why this is not feasible ie administering a vaccine which could potentially cause anyone to die suddenly is not feasible.
1 - Either millions and millions of people would drop dead like flies or no one would. Right? Otherwise, how could a deadly vaccine just kill 10,000 people and not affect 6.5 billion other people? If it were deadly, then literally millions and millions of people would die, not just a random few. Most of us would die, which ain't happening any time soon.
2 - Mark Zuckerberg cannot maintain his billionaire status if millions of people suddenly die. He stays a billionaire if millions of people continue to use Facebook. In other words, the elites need you around to keep the economy going and to stay filthy rich.
The elites need people around for various other reasons. If a war breaks out, they need people to defend their country. There is a war in Israel now. Did Israelis not take the vaccine? Do you think the govt would risk many young people dying? Who will protect their country during a war? The problem with this simplistic conspiracy is that you have no idea what complicated issues govts need to deal with because you have never run a country. Do you realize that many countries have a population decline? Do you think govts would risk administering sth that kills people randomly when they already have problems maintaining healthy population levels? If population levels reach unhealthy proportions, societies collapse.
So I've given reasons why your conspiracy is not feasible.
However, I don't discount that there is a possible future scenario where this sort of conspiracy may have validity.