You don't even need a knife, you already have several of them - spinning. http://www.dailymail.co.uk/news/article-2413231/Roman-Piroze... - "... was pulling a stunt with the chopper at a Brooklyn park Thursday afternoon when he lost control and the RC device flew into his head. The blades sliced off a piece of his skull and slashed his throat."
I'd never thought of stabbing someone with a drone. How much pressure does it take to puncture the average human's skin? How fast can a quadcopter fly? How much does a steak knife weigh? We can build the future weapons of war...
Less than you think unless you hit a rib or bone. But if you are really nasty just put pressure operated container that fires a poison into the body http://en.wikipedia.org/wiki/Bulgarian_umbrella style. Depending on the payload they won't save him even if he is in ER the moment he gets shot.
But they'll probably buy larger drones, take out some parts /weight and replace it with c4.