I actually had an in depth conversation on this topic with a military historian (an American). He explained that the military history of USA was born in the "Indian Wars" which were genocidal, hence No military culture of POW was ingrained in training. Remember that a 40 year old senior officer in WW2 would have attended Academy in say 1925, and it became illegal to kill Indians around 1908 in USA.
I have personally spoken with veterans who executed Japanese and German prisoners. In both cases they explained it was inconvenient/ impractical to hold them for logistical purposes, so they shot them. Both explained that as young soldiers they didn't mind as they hated the enemy but as years went by they began to see their victims as humans. If you really want to dig in, find a historian that digs into regimental diaries and statistics. You might notice that towards the end of WW2 USA collected and did not kill vast numbers of enemy but not very many in the first months after landing. BTW notice I prefaced my earlier comments on "based on direct interview". But, there is some shame in the military culture. An example of this manifesting can be seen in the movie "saving private Ryan", where the movie goes to lengths to show why they should have murdered a hapless German soldier who they show kindness to but instead goes back and joins the SS (see they should have murdered him). On a lighter note I remember a story from. Dutch man who took several Japanese prisoners at a remote observation point. He had no way to bring them to his side and they couldn't walk well as they were thin and sick. He had them burn their rifles and ammo, gave them malaria pills, and told them to burn all their radios and most of their clothing and all their boots. He said in a few days people would bring food but only if they would formally surrender to him. He told them they would go home where they were needed to rebuild, to fight on would in fact be cowardly. They all did, and indeed waited for later arrest.