Answer
if it's a stealth attack , i.e. if the enemy is unaware of your presence then a stab to a vital organ or slicing the wind pipe (trachea in the neck) will end it in seconds. In fact it is proven that it's easier to kill with a knife than a gun in close counters (but I don't buy it …personal opinion).
If the target is aware then the knife bearer will definitely require some training to go through the defenses …..I suggest slashing wildly which make it tough for the person to defend
Some fatal regions for an easy kill are
Neck (almost anywhere does fatal damage but prefered is the region under Adam apple ,a deep cut or stab will do the job
Brain ….a stab at the underside of the back of the head where the spine goes in to the skull…..will give a sudden kill ,this region is just above the neck and has a curve to it. ( Full Tang knife is recommended or a good, strong and sharp one)
Heart ….it is quite difficult due to the ribcage but can be exicuted with skill . Heart pumps blood to all the organs ..therefore it is a guaranteed kill
Not necessarily a easy kill but attacking all Internal organs i.e. consecutive stabs to intestines , Liver , kidney, oh especially the spleen…will be a slow yet painful DEATH
i can help hide the body
best advice from the best killer