There is a reason why for example in the state of Missouri we differentiate between a 4 inch or less folding blade and anything longer. You might be able to kill someoine with a blade like that but it is extremely difficult and one can easily defend themselves from it without the need for deadly force. The long the blade, the deeper it goes and obviously the easier it is to do serious bodily harm.
In case you're curious here is the statutory definition of a knife in Missouri:
(12) "Knife", any dagger, dirk, stiletto, or bladed hand instrument that is readily capable of inflicting serious physical injury or death by cutting or stabbing a person. For purposes of this chapter, "knife" does not include any ordinary pocketknife with no blade more than four inches in length;