The Switch

Everyone has a range of skills and abilities.  That's too obvious to write, really.  Let me give a concrete example.  I have a couple of friends I play with.  Health issues aside, I think one has slightly more combative skill than me, one slightly less.
The difference in skill doesn't matter a damn bit.  If either of these guys chose to go lethal, there would be too much damage done in the initial instant for me to recover.  At that level (and it doesn't actually take much skill) the ability to flip the switch is everything.
It's an easy thing to say, it's easy to believe that you could decide to flip the switch, but most don't.  They either freeze entirely or they stick in Monkey Dance/dueling/fistfighting/social conflict mode.  Those are all the same thing (as are sparring and sport at any level).  They are qualitatively different than what I'm talking about.
The first time you slaughter an animal, maybe every time, there is an emotional content to it, but it is not the same as fighting a person.  If you are going to chop the head off a chicken, you don't need to get mad or be righteous or make it personal or go through any steps.  It's a chore.  You just do it.
Most people cannot just kill a person.  They need a reason and a justification (two separate things).  Most need a ritual of blame and challenge and it's not even enough to take the guy out but they need the victim to see who did it.  That's human on human stuff.  When you flip the switch all of that is just wasted time and energy.  It also makes you very difficult to predict or defend against because humans depend on the ritual in other people to keep the violence social.
There's a lot in that last little thought, including the 'werewolf problem:' If someone can flip that switch with no warning, one of the deepest social conditionings, what else can they do?  What can stop them?  Can anything control their behavior?
I think the reason a lot of skilled martial artists don't ping my radar is that I don't sense this switch in them at all.  One told me years ago about the beast inside him, how he had once felt this awesome killing rage.  He feared that emotion.  What I heard was that even under extreme rage and fear he did absolutely nothing.  No matter what skill he had, he was and never would be any threat to me.
The Hulks of the world ("You wouldn't like me when I'm angry.") don't scare me.  They aren't a problem.  You will almost always see them coming, they will give plenty of warning and they will fight like anyone else in a Monkey Dance.
It's the ones who can approach violence as an unpleasant chore, something to get over with quickly and efficiently who are dangerous. The ones who can decide to flip the switch.  Who know the difference between fighting another person and simply slaughtering them and can choose.
Note:  Slaughtering is an example because it is something we do to animals and can relate to.  A professional can also flip the switch for handcuffing or taking some one out at a non-lethal level... I just didn't want to cloud the example with all you twisted people combining livestaock and handcuffs in your imaginations.