Who prevents him from buying an atomic weapon and setting it off for a meme stunt or internet points ?
You have to be joking.
Nearly every military in the world. Countless regulatory agencies. Intelligence agencies the world over. It's pretty much known that the US made stuxnet to kill one country's nuclear program. Do you seriously fucking think they wouldn't stop a single billionaire?
There's also the fact that even he's not that insane, and any other billionaire out there who wouldn't want the effects of a nuke going off to get in the way of their own shit.
If you were talking a dirty bomb, that might be within his reach. Buy some mines in third world countries, mine up some material, strap it to a conventional bomb. That's also many orders of magnitude less severe (while still horrific). Also, most mining rights in areas with worthwhile radioactive material available have already been bought up by other entities with similar financial levels of backing.
Actual nukes require quite a bit more than just an explosive and some radioactive material to build anyway, and things like nuclear material refinement facilities are quite easily visible from satelite imagery. They also require specialized hardware that is closely monitored. Sure he could pay to reverse engineer and/or get it built. Good luck keeping that secret for as long as it would take.
The man's a living embodiment of a chode with a diamond studded piercing. There's plenty of shit to be upset at him about, or worried about, without getting anywhere close to this absurd. I sincerely hope that you weren't being serious.
If you want shock factor, talk about the slave mines his family wealth comes from, and the slave mines where we source lithium from for EV batteries. Talk about the high frequency of using child soldiers as security for said mines, in addition to the child slave labor.
Talk about the highly likely intentional killing of Twitter by Saudi Arabian government's investment into Musk as a retaliation for the Arab Spring and as a way to further control rapid information dissemination during crisises.
There's real reasons to despise him, going for such extremely ridiculous exaggerations only hurts the point you're trying to make.