why do people allow themselves to be executed?Watch
But since these POW know they are going to die anyway, why not make a last ditch struggle to save themselves? like charge at the gunmen, or run, or scream profanities at the executor, Instead of waiting for their fate?
But probably know that if they tried something they're going to get slowly tortured to death instead of a painless execution.
Tbh the director is playing a few tricks on you there - at the start of the clip there were a bunch of cavalry who are presumably still hanging around... also the rifle guy is further away than he looks - dunno if the reload time is accurate for that type of rifle but he seems to get a couple of shots off faster than they'd cover much distance... and the surrendering guys were probably not expecting to get shot and it took them a second or two to react to the unexpected turn of events.
Taking prisoners and treating them fairly well has advantages - enemy soldiers will generally surrender to you more quickly which is in fact better for everyone. Officers might give you useful information about strategy, strength and location of other units etc. if you don't murder them.