Whether it is during an act of war, or whether it is done through a prolong fight or through a high speed chase, when one person or one soldier or one perpetrator surrenders to another person, or soldier, or policeman, such surrender does not always result in the party so surrendering being safely escorted to a place of confinement, but rather can end instead with that person that has offered to surrendered, being killed, or essentially executed.
Not too surprisingly, firefights, chases, and conflict of all sorts, increases the stress levels within a given body, which for most people means the increase of adrenaline within that body, culminating with that corresponding adrenaline rush, and those that are pumped up with adrenaline coursing through their veins, are often at odds at being able to subsequently easily calm down, even when a given stressful situation has materially changed. This means that when one party surrenders in defeat that the victorious party does not necessarily readily become magnanimous but rather can see such as an opportunity to finish the job that they were prepared to do when the fight so began.
So that, while a person in good control of their faculties, would normally not kill another person in cold blood, someone that feels aggrieved or has seen their fellow soldiers or friends die, or have been under fire or in mortal danger, often does not, upon the surrender of their enemy, simply return to being a reasonable and rational human being. Additionally, if the enemy does not look like the other person, but rather appears instead to be something other than a full human to the other person, or essentially that person has been demonized, than the mercy that the surrendering person might reasonably expect, is going to be difficult to bring to fruition.
Those that surrender typically do so, when they know that continuing to fight or resisting within the fight, is futile, because they are outnumbered or because they are out armed, and rightly believe that their only good option is to surrender to the other party. The problem with such surrender, especially when it is done in the heat of the battle, is that the other side, has not yet had the chance or opportunity to dial down their rage, or to neutralize their killing mindset, or simply do not care to do so at that time, and well might feel that they must kill in order to bring completion to the job at hand.
The fact of the matter is that with the exception of psychopaths and sociopaths, killing other human beings is not the natural order of things, so that, in order to accomplish such a deed, this requires a mindset that often looks upon the other person as something less than human, and therefore the surrender of something that is less than human, still allows the one with the power to have the superior position of apparently being able to justly kill that which has aggrieved them, because they are literally at that time and place, their judge, juror, and executioner. So that, those that surrender in the heat of the battle, while deserving of mercy, do not necessarily receive that, because those that are human, have forgotten that they are, and have morphed into, instead, for a period of time, a killing machine.