I think genuinely dangerous ideas are hard to come by though.

Daniel Dennett wrote a book called "Darwin's Dangerous Idea", and when people aren't trying to play down the basilisk (i.e. almost everywhere), people often pride themselves on thinking dangerous thoughts. It's a staple theme of the NRxers and the manosphere. Claiming to be dangerous provides a comfortable universal argument against opponents.

I think there are, in fact, a good many dangerous ideas, not merely ideas claimed to be so by posturers. Off the top of my head:

• Islamic fundamentalism (see IS/ISIS/ISIL).
• The mental is physical.
• God.
• There is no supernatural.
• Utilitarianism.
• Superintelligent AI.
• How to make nuclear weapons.
• Atoms.

Ideas like that usually don't pop into the heads of random, uninformed strangers.

They do, all the time, by contagion from the few who come up with them, especially in the Internet age.