Knife stabs shouldn't kill when you're facing someone who's shooting you. Knife stabs should only be instant kill when you sneak behind and back stab, or like Battlefield when you grab them and slit their throat. In Counter Strike the knife is balanced so it's only used is specific scenarios, and not general use when you're already holding an assault rifle.

well if the guy its unloading massive amounts of firepower unto you it is, plus its far harder to grab someone and stab them with no resistance when the other guy can completely see you.