Though that´s the thing. more than 50% of the time, a knife isn't going to kill at all due to the amount of wind resistance a knife would have unless it's a specially made one, not to mention due to the fact it goes end over end and the further it goes, the less velocity it will have by the end. Infact, they shouldn't be treated as a killing weapon unless you plan on stabbing. Better to use it to distract the person. HE is definitely going to flinch and hesitate and then use that opportunity to attack. Hell, that's what shurikens were mostly for, not for killing