The purpose of patents is to encourage the complete disclosure of inventions so they can be used & improved later by others (or even worked around with better inventions now), in return for temporary exclusive rights. Patenting evil inventions might be a way to advertise and accelerate the proliferation of evil ideas…
It kind of gives the lawmakers a head start. If you release some evil product today, then you're going to use all of your money to fight any regulation or legal disputes. But if the idea exists and there's only the threat of it being used, then none of those fights happen, but people will still ask their lawmakers to ban it. And they have decades to get it done.
Imagine if I had patented "System for using the Internet to distribute misinformation regarding respiratory viruses, so as to maximize deaths during a pandemic." Facebook would show up later and already be illegal, and a lot less people would have died of COVID.
I doubt the example in your second paragraph is remotely plausible. But I can see how someone might come up with a better example.
Btw, violating a patent isn't illegal in the criminal sense of murder being illegal. As far as I know patents are more about civil liability, and you'd have to sue Facebook for damages, don't you?
That would make doing business quite a lot more expensive for Facebook, of course. But wouldn't ban what they are doing outright.
