Most martial artists would not survive a real-life street encounter. In their arrogant overconfidence gained from training in strictly controlled one-step and free sparring, their survival instinct has been dampened. They have become taekwondo athletes, like dancers. They know the techniques, but they lack the "killer instinct" necessary for survival.
For survival, it's a must have
Don’t think this "killer instinct" is unnecessary or that you could never develop that kind of instinctive, animal-like focus. When a mother is forced to protect her child from harm, she instinctively enters this mindset. When you are so angry and indignant that you no longer care about the consequences, you are also in this frame of mind. You can, and you must, learn to consciously cultivate this mindset. You must be prepared to fight as if you were about to die at any moment. This overpowering desire to survive at any cost is what self-defense is about.
Self-defense has nothing to do with breaking boards or kicking and punching while making loud yells. It is a state of mind, a type of consciousness, a peculiar awareness that keeps you alert and able to respond to any possible danger. The actions are merely physical movements, which are useless without a survival mindset.
In combat, you cannot worry about the educational background, religious background, economic background, race, gender, nationality, etc. of your attacker. You must have a combat mentality that can channel destructive forces that exceed those of your aggressor. You must be a cold, vicious animal; free of sympathy, fear, anger, apprehension, and ego. You must possess a killer instinct.
Nowadays, it's considered inappropriate
Some martial arts do not stress or even encourage a combat mentality. It is concerned uncivilized and barbaric. For some, it violates their religious and philosophical beliefs. Others replace it with a competitive, sport-oriented mentality that lacks the brutal and aggressive characteristics necessary to neutralize a crazed criminal attacker.
Everyone is born with a killer instinct; this how humans have survived to this day. In less civilized times, those without it did not survive. It is stronger in some than in others and many have been taught to suppress it and they try to justify their inaction as being the "right" thing to do, at least until they need protection, then they want the ones with the killer instinct, the warriors, to protect them.
It must be controlled
Some manifest the killer instinct inappropriately in blind rage or haphazard fury. They are driven by a raw killer instinct that is inferior and undisciplined. Their energies are poisoned by emotion, resulting in poor techniques and tactical errors.
On the other hand, warriors tap into the emotional calm and mental clarity of the killer instinct, and, by using virtue and courage can disperse deadly destructiveness appropriately, without the constraints of emotions. This may sound paradoxical or extreme, but a warrior must be virtuous, and yet capable of unleashing controlled viciousness and brutality. Napoleon once said, "The moral are to the physical forces as three are to one."
The killer instinct is predicated on being emotionless. A warrior must temporarily eliminate fear, anger, remorse, and ego from consciousness. From childhood, we are conditioned to express feelings for ourselves and others. Humans are expressive beings, crying when hurt, laughing when happy, and yelling when angry; however, a warrior must remain emotionless during a violent confrontation because emotions create indecisiveness and dangerous tactical judgments. Control of emotions also prevents anger from weakening the warrior.
A warrior must control and suppress anger. Anger is a useless emotion that wastes energy and causes poor judgment. The famous satirist Pietro Aretino said, "Angry men are blind and foolish, for reason at such time takes flight and, in her absence, wrath plunders all the riches of the intellect, while the judgment remains the prisoner of its pride."
A warrior must not fear death or physical disfigurement. Some see fear as a positive defensive attribute, believing the "fight or flight" syndrome will help defeat the enemy. However, if fear causes inaction or insufficient action, it is detrimental.
A warrior must be vicious, capable of extreme violence. Some consider this revolting, at least until they need a vicious person to protect them. A warrior must be more vicious than his adversary. Modern soldiers are supposed to be guided by the Geneva Convention (the rules of war); as if you can impose rules on the enemy. As exhibited by the 9/11/2001 attack on the United States, the enemy has no respect for the Geneva Convention. To convince the enemy to stop its aggression, you must be more ruthless than they are, or you will fail. If you do not believe it, just look at the results of wars, both ancient and recent.
A warrior must have a unified mind that is free from distractions and fully focused on the enemy. Distractions are derived from two sources: internal, where the mind wanders or panics; and external, such as an adversary attempting to "psych you out" or environmental conditions such as weather, lighting, terrain, etc. acting upon your senses. A warrior must control thinking, situational awareness, adrenaline manipulation, physical mobilization, psychomotor control, tunnel vision, courage, tactical implementation, breath control, pain tolerance, and habituation to violence.
Everyone has a killer instinct. In some, it is weak; in some, it is strong. In some, it is suppressed; in some, it is expressed. For the warrior, the killer instinct must be nurtured and cultivated into a controlled response that acts for the good of all.