Do you know why I use a knife? Guns are too quick. You can't savour all the little emotions. Y'see, in their last moments, people show you who they really are.
Usually I see a look of shock when I cover their mouth from behind with my hand, and tears when I stab them in the heart by bringing the knife toward me from the front. It doesn't tell me much since they don't see me. If they see you, than they reveal their emotions like cowards begging.