I don't know about "take them to the rear" being a dog whistle for "shoot them". For one thing, prisoners can be a valuable source of intelligence. (Many Japanese who surrendered were seething with resentment towards their officers, and willingly gave as much information they could.) The following snippet is from the memoirs of Major Dick Winters (Band of Brothers) T/5 Leibgott's parents were Austrian, and he spoke fluent German. He hated Nazis so badly that his fellow soldiers assumed he was Jewish, which he was not.
My casualties were one man dead and four wounded. Tech/5 Joseph D. Liebgott had been slightly wounded in the arm, but he was ambulatory so I assigned him the mission of escorting seven German prisoners to the rear. Liebgott had earned the reputation of being one of Easy’s best combat soldiers, but we had all heard stories that he was very rough on prisoners. Liebgott was one of Easy Company’s killers, so I deemed it appropriate to take a bit of caution. When he heard me say, “Take the prisoners back to the battalion command post,” he replied, “Oh boy! I’ll take care of them.” In his exuberance Liebgott stood up and paced back and forth and he was obviously very nervous and concerned.I stopped him in his tracks. “There are seven prisoners and I want seven prisoners turned over to battalion.”
Liebgott was highly incensed and started to throw a tantrum. Somewhat unsure of how he would react, I then dropped my M-1 to my hip, threw off the safety, and said, “Liebgott, drop all your ammunition and empty your rifle.” There was much grumbling and swearing, but he did as I had ordered. “Now,” I said, “you can put one round in your rifle. If you drop a prisoner, the rest will jump you.” One of the German prisoners, an officer, evidently understood this exchange. After he understood my orders, he relaxed and sat down. Liebgott returned seven prisoners to battalion headquarters that day—I personally checked with Nixon.
My paternal grandfather served as reconnaissance with the Trailblazers in Europe. He never talked about the war when I was growing up, and only in his last years did he talk about some of his experiences. One thing he did related was having German prisoners, captured for interrogation, shot trying to bring them back across American lines. It bothered him greatly.
He also spoke of having to kill German soldiers who were just kids. He was born in ‘24 so he himself was only barely in his 20s.