How? First off the story from the movie was more like a rumor between the soldiers as to what happened, since apparently no one actually saw it.
assuming the gif is even true(because it was just one of the rumor flashbacks) Let's use our heads here. Context. A bunch of PARATROOPERS behind enemy lines. Outnumbered to fuck, supplies and shit scattered everywhere. Cutoff from everything for the foreseeable moment.
SOMEONE took some prisoners. more than they can handle, for reasons stated above. Before they move out to accomplish one of their critical objectives, they kill them off. Possibly inhumanely. Possibly not. Maybe he didn't kill anyone at all. No one even knows.
Inhumane or not,
Was he supposed to hand cuff them, and leave them there with no one watching them? Was he supposed to let them go? Did you watch saving private Ryan where they let some German pow guy go and he came back and killed the same guys that let him go? Should they Leave them for the French partisans to get? Leave them for the Germans to recapture?(How did Germans treat recaptured POWs... Probably not that well I'd imagine if they were somewhat like the Soviets)
Should he turn them into the local French police department? I mean What do you do in that situation? It's a war, not pretty but it was the most logical thing to do ignoring the way it was portrayed in the movie.
I don't mean to turn this into a WW2 morality discussion over a gif, that didn't even show what you're talking about, but you triggered me with that.
The prisoners were POWs but charged with Treason- if you switch sides during wartime & you are caught out your life is forfeit. That's what the whole point of the conversation that the young feller had with the German POW before the execution- the German was actually American.
Same thing happened to Alsatian Germans that choose to fight with the Wermacht. Summary executions.