Before there were modern weapons, or various other weapons of war which allowed a person to target and to therefore harm or to even kill a moving target from a distance, then running away and fleeing from a situation in which one's position was no longer considered to be tenable, was a reasonable thing to act upon. For basically, to outrun an opponent, or to flee from a negative or a dangerous situation, or maneuver through fields or forests in a manner that someone else would have a very difficult time in tracking an individual, was a viable as well as a sensible way to live to fight another day.
So too, fleeing had its place also as a way to get back to the home base and to thereby inform the community or the troops, that there was danger on its way, and therefore to allow that community or soldiers to better prepare and to better defend against such an intrusion. All of this makes sense, and certainly in warfare it was common for reconnaissance to be practiced as well as intelligence gathering to be conducted so as to best prepare strategies and to have foreknowledge, in which those doing such reconnaissance, did so, recognizing that the objective was never to engage with the enemy, but to collect information and then essentially flee back to safe territory.
In today's world, each side has a multitude of weapons that have the ability to kill or to harm others from a distance. Additionally, in many combat situations, ultimately one side overcomes the other, and when the side that is being vanquished, recognizes such, then a decision must be reached by those combatants as to what is the best avenue for them to take, of which, within those choices, one can retreat, or flee, either in a planned maneuver or chaotically.
It may come as a surprise to some, but when one side is no longer fighting, or is sporadically returning fire, the other side, cognizant that the danger to them has been reduced considerably will in most cases, increase their fire power and will chase down their opponents, not so much because they have a blood-thirst to do so, though, they might; but rather, because like a bully that knows that their opponent has not the means or the size to fight effectively back, these soldiers recognize that when they are no longer in mortal danger, that to follow their orders from their commanding officer to engage the enemy is now a lot easier to adhere to.
In point of fact, those that turn their back and flee, are inviting targets for any soldier or anybody in policing authority, to shoot at, because when the rules of engagement have already been clearly set, then fulfilling them when the situation so fully favors the pursuer over the pursued, is something that the pursuer is going to do; because at the end of the day, almost all soldiers and policing authorities are an integral part of a chain of command, and when that command allows them to shoot to kill, often both their accuracy and shots so fired are far more active when they face little or no endangerment to themselves.