From Krav Maga Systems' perspective, to be effective at protecting ourselves, the 'killer instinct' is a quality that we all need to have. It is important to understand that the 'killer instinct' in this case does not refer to your ability to completely end someone. It is the 'x-factor' - that one thing that could be the difference between you surviving or being the victim.
Often when students first start out in Krav Maga training, particularly students who are new to the world of personal protection, they have a few emotional/mental barriers that stop them from switching into the killer instinct mode (or more accurately, being comfortable with it).
In the modern society we live in, most of us are pretty civilised. We respect each other's right to be safe, we generally avoid starting conflicts and do what we can to help each other out.
So when you move from that world and step into the reality of personal protection and Krav Maga training, it can be a bit unnerving to start thinking and behaving in a way that contradicts these social norms. Some of us are simply uncomfortable with exerting physical dominance over someone. This could be because we fear any potential legal consequences, we may not like inflicting pain on another, we fear what they may do to retaliate, or we simply don't want to experience pain: all of these are natural human fears.
Suffice to say, though, taking the time to consider these consequences in the heat of an attack is not going to do you any favours. Adam Sinicki from the-biomatrix.net makes a great point by saying "...they [the assailants] have learned the art of intimidation, they have let go of their fear and they have that 'killer' instinct that most people are lacking." (1)
While you're standing there looking at your assailant, worrying about avoiding all these consequences they are dead-set on being the one to dominate and get what they want. You have to beat the assailant at their own game (psychologically) - if you are focused on consequence, and they are focused on domination, you have already lost. You won't be expecting it when it happens. They already have the upper-hand by initiating the situation, so you need to be able to snap into that mindset at will.
This is why, in training, it is not good enough to go through the motions. It is not enough to mark your targets, do a couple of palm strikes in the air and then move away from the 'danger'. You need to get used to channelling your inner predator. Again, it isn't about turning into a homicidal maniac.
Developing your killer instinct will help you overcome your fear/freeze response. Think of it as your guardian angel, it knows what it will take for you to survive, so you gotta get comfortable with this side of yourself to harness the resources you need in those moments of chaos.
Put yourself in that mind space while you train. Remind yourself why and what you are training for. Sure, you may be buddies with your training partner, and you don't want to harm them (that is the no.1 rule in Krav Maga - don't get hurt, don't hurt your partner), but when you're in the training environment, you're training to know how to survive - it isn't just about punching and kicking. Training needs that element of dealing with the realistic intentions of the attacker and the defender.
As Lee Morrison says: "Control of negative emotion leads to the access of skill under pressure and this starts with the way you think, or your perspective. This, as I said before affects the way you feel, the action you take and the results you ultimately get." (2). Use the training environment as an opportunity to acknowledge your fears and switch into the proper mindset. Do you think you can be worried about all the things that could go wrong AND be able to take effective action at the same time? Under the stress of adrenaline, and fear, it is unlikely.
Be focused on what you're going to do to the assailant in front of you, not on the consequences of what you could be facing. "Their [the assailants] perspective of the pre-fight event will be the same or very similar....whether it’s a seasoned street thug, a violent sociopath or a Spec Operative..." - that is the mindset that comes with the killer instinct.
We cannot stress enough the importance of practicing this mindset in training. You have to train your mind to block out any fear or doubt. Make yourself mentally unbreakable - this means you have to learn to accept unpredictable circumstances (so train to solve unpredictable problems), become adaptable and become a master in the attitude of success. Don't let yourself settle anything less.