scary smart
A demonstrated level of superior mental intelligence, accompanied by a high I.Q., beyond that which people of "average" intelligence find comfortable or can understand, often to such a greater degree that it leaves them feeling intimidated or even somewhat frightened.
There is "smart," there is "really smart," and there is "scary smart"—so smart that you are glad the person uses their mind for good and not to rule the planet for their own enrichment. ~Professor David Tulloch, Rutgers University
Scary Smart
A person who is so intelligent that their ideas are frightening; an evil genius.
The person who planned the nanochemical attack was scary smart.