The knife is the most powerful weapon. You cant just swipe at people though. The more you swipe and miss, the less powerful it becomes. So, sneak up, take target, and actually have good enough aim to kill someone, and it's a one shot kill fest. Remember, the count resets after every kill, so you can mow down enemies.
the knife is a powerful weapon, make a quick key for it, or learn how to use it before saying such things.