In the comments, the topic of how stress affects reactions came up. It wasn't really germane to the topic of mercenaries, but it is a factor in any emergency situation.
The perception of danger has physiological effects that are not under an individual's control. Individuals vary in how their individual physiology reacts--how much adrenalin it squirts out, for instance. And the perception of danger, for many people, is related to experience and training, and thus--to some extent--is handled by consciousness. Thus individuals react differently--physiologically and emotionally--to the same stimulus, the same stressor, the same scary experience--from both innate (primary) and experiential (secondary) causes.
In general, for most people, whatever their starting level of response, experience and training move more of the response from unconscious/uncontrollable into conscious control--into an area where they can think, assess the threat, use trained responses. The person who first faces a public performance--whether it's to play an instrument, give a speech, sing a song, make a critical play in a sport--is much more likely to "choke" than the person who has done it many times before. That's why child musicians, dancers, speakers, athletes give their first performances to smaller, friendly audiences--it helps them over the initial performance anxiety. Then by the time they're in the Olympics, or an international music competition, their parents and coaches hope they're immune to that kind of thing, that the training will take over. That's what training is for.
Training for dangerous professions--military, law enforcement, EMS, firefighting--is intended to provide the trainee with such overtrained responses that he or she will act to the training in highly stressful situations. This is easier for some individuals than others, due to their physiology, the way they're wired, and to their previous experiences. (Churchill, for instance, reported that his first experience of being under fire was exhilarating.) Some people cannot help throwing up when they smell vomit; some people faint at the sight of blood--these people are not suited for EMS work, obviously. Others never have a problem staying calm in an emergency, and get the shakes before (on the way to the problem) or after (when it's all over) or not at all. Every emergency reveals some individuals --including "untrained" individuals--acting calmly and rationally in the face of deadly force (be that force manmade or natural, like an earthquake.)
While it is unrealistic to expect that all professionals will perform perfectly in dangerous situations, it is quite realistic to expect them to perform better than 85-90% of untrained persons. If they don't, either they're fundamentally unsuited to the demands of their job, or they lack sufficient and correct training. Correct training will discover most of those who are so unsuited (not all--there will always be sad discoveries on battlefields and in firefights) and will bring the rest to the highest possible level of function. It is particularly important to ensure that the training of those allowed to carry weapons is complete and that those persons are capable of assessing a situation and responding out of reason, not out of minds clouded with fear, panic, or anger. Being out of control emotionally to the point of being unable to use one's skills is unprofessional.
When a trained individual acts below the level of training, then either the individual was passed through who should not have been, or training was insufficient. Or the individual has a bad attitude, a belief that he or she doesn't have to respond as trained, is outside the rules (another person who shouldn't be in that field.)
When I was in the local EMS, I worked alongside both volunteers and professionals in a number of critical situations, some personally dangerous. I did my hospital training for both EMT and Paramedic at a military hospital. I was able to observe that some individuals, at the same training level, were never able to control their reactions and remained panic-prone, while others were calm and capable of retaining training skills and responding thoughtfully no matter what. I had seen the same thing in other venues. I was also involved in training others, and learned to recognize which ones were going to have more, and which less, trouble when they got out into real-life situations. So when I say that good training, combined with an individual capable of benefitting from it, produces someone who shows consistently better performance in emergencies than average, I do know what I'm talking about.
That is the difference between the well-trained professional and most of the amateurs (except those with exceptional ability.) The trained professional does not panic, does not forget his/her skills, is able to keep thinking even under great stress. Like the off-duty policeman at the mall in Salt Lake City, the professional is able to assess the situation and make sound decisions in those situations. And the exceptional amateur, like the professor at Virginia Tech who saved his students at the cost of his own life, can do the same.
Unfortunately (though in accord with human nature) each group judges the other more harshly than its own. Law enforcement personnel routinely expect rational, calm behavior from untrained amateurs in situations where they themselves are hyped on adrenalin and making poor decisions. They exacerbate situations by being loud and acting (whether they are or not) furiously angry, without apparently understanding how their behavior elicits undesirable behavior in others. This is not true of all law enforcement personnel, but it's clearly true of some. Similarly, the average citizen wants law enforcement personnel (and firefighters and EMS personnel) to act exactly the way they should (or the way the citizens think they should) without regard for their inevitable limitations.
Still, citizens have a right to expect that professionals will act professionally nearly all the time. That professionalism includes the ability to control emotional response of all types: revulsion, fear, anger, etc. Doctors and paramedics aren't allowed to scream at, slug , or kick someone for making an obscene gesture at them, or cussing them out (and it happens, believe me!)--neither should police (and that happens--happened in Austin just this past weekend.) I personally think that the "reality" cop shows have encouraged a loosening of professional standards--encouraging "striking" behavior rather than quiet professional behavior, but I could be wrong about the cause.