I can see merits in both sides. "We" were supposed to be on the side of "Right" in that war and not capable of atrocities, unlike the "Enemy"; on the other hand, street fighting/house to house clearing is nerve wracking; is death waiting behind that next closed door?
I am sure many instances occurred as happened to our new friend's late grandad; when nerves are tried beyond normal human experience and every moment could be the last. The instant reaction of that young soldier to danger might have been followed by regret, and relief at surviving another encounter. The majority of GIs' were citizen soldiers,"soldiers for the working day" and not long service professionals. As collector/historians comfortably removed from that war several decades past, although I still remember the bombing and civilian casualties, we should strive for balance in our judgements.
Not intending to preach here, I am 74 years old and it is,jmho.
