The most dangerous thing discovered or invented.
The most dangerous discovery would probably be fire. Humans have burned so many billions of square kilometers of woodlands, burned animals, enemies and anything else they came across. Problem with fire is, that it is not only deadly, but also spreads on itself. Once it gets out of control a fire can change the ecosystem of an entire region for the next fifty years to come.
Invention is much harder to properly assess, however if we were to take "dangerous" as "killing potential" I would have to say the atomic bomb in combination with ICBMs and the industrial capability to mass produce both.
We could probably kill everything on earth with those if we truly wanted to.