It is your killer instinct which must be harnessed if you expect to survive in combat. It is a hard heart that kills. If your killer instincts are not clean and strong you will hesitate at the moment of truth. You will not kill. You will become dead, and then you will be in a whole world of shit because you are not allowed to die without permission. Do you understand?
Gunnery Sergeant Hartman (Paraphrased)