Could nano technology cause an extinction level event?
That would depend upon how the science is used and whether or not nations that have it, decide to weaponize it.
In all likelihood, nano technology should be good for humanity, giving us new and innovative medical treatments, helping us to develop other new technologies which could be used to hole us get out into space or deep beneath the oceans.
If used for untoward purposes, any technology could be considered bad and a danger to humanity.
That being said, could the use of nano technology cause the sixth extinction?
There are only two ways this might happen - one, as mentioned, weaponizing or two, where the nano-bots gain sentinel in some way and decide we are superfluous.
For now, the possibilities for either are pretty small but technology jumps ahead in leaps and bounds so watch this space!