You need to be able to see a man's face that cannot see you and kill him. Watch him die. See his brains splatter out the back of his head and do it again and again and maintain the ability to look in the mirror and sleep at night. Got that stud?
"The purpose of fighting is to win. There is no possible victory in defense. The sword is more important than the shield and skill is more important than either. The final weapon is the brain. All else is supplemental." - John Steinbeck