Come on... endlessly wealthy and wants to do harm? There's only one real, obvious answer: Engineer a plague, or a series of plagues targeting food crops, livestock, and humans. Make something as contagious as measles that kills a third of people reliably, spread it multiple global focal points at once, and you could topple the world pretty effectively. If you attacked the economy and food supply at the same time I don't think there would be any coming back from that.

It would be insane, an apocalyptic fantasy that few could afford outside of (hopefully somewhat) accountable or at least pragmatic governments. But a very rich person could afford it.

And as much as information warfare is terrifying, biological warfare is much worse. I get your "it's already here" point, but lets really take the question at face value and run with it.

edit: Consider that a group MUCH less wealthy than Elon Musk was able to experiment with nuclear weapons, and managed to carry out a sarin attack.