I mean really think for a second what is the worst possible future besides us all being dead
Dying in an extremely excruciatingly long and painful death?

Like, being eaten alive, cell by cell, by nano-bots. Personally, I think that if we develop any such nanotechnology, we should keep it mechanical, so that we can EMP them if they get out of hand. Organic nanotechnology, is a whole lot more dangerous.