Isn't the point of Proof of Stakes that you hold some amount of coin to exert that control. If someone or some group get majority stake, doing anything nefarious would result in crashing the coin value, and thus nuke their own coin value?

Yes, but thinking this is a safeguard presumes the exchange value of the coin must outweigh any other concerns the actor has.

Yes, but they can make it up with some other gain in value. For example if you're a US state agency tasked to destroy the Chinese economy, you might overall benefit from buying a ridiculous amount of Chinese money and then setting it on fire, if it means everyone else's Chinese money catches fire too.