what is a scary idea if it was invented by people and got made?

what is a scary idea people can create in real life?
I would be frightened at the idea of hypnotism. and time traveling. I think some things are too dangerous to have in this world worse than guns. although we already have so many things that are equally bad, the atom bomb,any chemical weapons and diseases that can spread. I think the diseases are the scariest because you suffer the most. I know someone who is so paranoid he is claiming that the government keep the worlds deadliest diseases. im guessing to be able to study them, but if some bad person got in he could release it on the world, i don't think it exists though.