Could the term 'killer instinct' be synonimous with "commitment" or "determination?"
By that I mean the willingness to do what needs to be done in order to accomplish the goal of stopping the threat, escaping the danger area...what ever. If you are truly 'defending yourself' within a reasonable limit, you have to tap into that 'determination' to accomplish the goal - if necessary because of the circumstances to the point of lethal force.
At different phases of training, I have seen people wrestle with "could I really do that?" with techniques, levels of force, moral quandries.... which I think is healthy. I wouldn't want people blindly willing to leap on someone at the smallest sign of danger.
I think educating people on the legal limits, helping them work through their personal ethics/morals/values so that they can CONSCIOUSLY say "Yes, I understand the consequences of taking or not taking this particular action" and make the choice that accomplishes the goal AND keeps them within their moral structure. WHen the fit hits the shan and you have to decide between things, usually it isn't between GOOD and BAD so much as BAD and BADDER because there are always risks, complications and problems but it is important to be DECISIVE and not kill valuable time dwelling on things, save that for after the moment. Tactical mentallity is different from reflecting on something, (call it "mental fluidity").
Not to be morbid but IMO Killing is as intimate or even MORE personal than sex because it is permanent. Even if it is all just theory until you come to that moement, having some understanding (physical training as well as internal examination) about how and what you do under stress helps you "see" and "know" what your potential for actions are. THEN you can start deciding on what to do about training yourself to do something else (behavioral/neural habit training to over ride prior habits/tendencies) or to enhance and build off of those already existent habits.
Are you an instinctual 'cocooner' and shut down? Do you get sleepy, freeze, laugh, cry, angry, growl, 'crazed', go on the offensive....?
Instructors talk a lot about what they 'teach' people, but it is just as important to set up 'personal observation' moments that let people get a glimpse of what they are like under stress as well.