You want at least four inches. Anything less, and you won't penetrate far enough.

"The objective is not to maim or incapacitate, it is to kill. There are two ways we can achieve this - two methods that we can use. Name one."
"Blood-letting"
"Yes, blood-loss is one. Vital organs is the other."

. . .

"From the front, we have access to most of the vital organs and vessels. In the neck, we can attack the carotid arteries, in the legs the femoral arteries. From the shoulder we can target the subclavian artery. For vital organs, there are the obvious - heart and lungs, and brain too, through the eyes. From the rear, we have the kidneys, and above that, the liver."

. . .

"The upper back of the human body is well protected. The traps and the shoulder blades act like a natural armour, to protect the vital organs and blood vessels. Luckily, from behind, we can normally move straight to a kill-move."

. . .

"Don't look at the back of his head, look off. See him in your peripheral vision. Stay low. Walk quietly. Don't cross your legs. Get close, but not so close that he can see you in his peripheral vision. When you're ready, switch the grip and move in."

. . .

"Maintain control of your target at all times. Grab his forehead or helmet, or you can cover his mouth to muffle any screams. You may want to disrupt his balance, like this. You see?"

. . .

"Aim for the hollow just behind the clavicle. And don't just stop after the first one. Keep on going until he stops struggling. Ventilate him."

. . .

"Reach over or under his arm and grab him by the throat. Pull him back to provide a counter-force while you stab at an upwards angle. Aim for the kidney and liver. Nothing fancy. You don't have to stab and twist like in the movies. To him, it'll feel like a hard punch to the kidney, and the shock and pain will prevent him from screaming. Again, control the body, and don't stop until he's dead."