joining the military are clearly known. In the case of WWII, young men and women enlisted because of the nature of and reason for that war. Once one is in the military, it's rare for those below the rank of colonel to really know the details of whatever battles or missions they are asked to undertake. In that sense, they are unable to choose between serving or not serving in a particular instance.
In some cases, the nature of some mission may become clear at some point. Atrocities occur when ordinary soldiers ignore the morality of their actions and do immoral things. Such atrocities occur in every war, sadly. Defining what is and is not an atrocity is a difficult matter, in many cases, and each person's definition is unique.
But, those judgments are most commonly made long after whatever happened happened. Hindsight.
I also served in the military, during the Vietnam war. As luck would happen, my service had no connection to any combat operations at all, but was related to the cold war, which was going on at the same time. I was posted to a tiny USAF base in Turkey, where the Russian language I learned while in the USAF was thought to be useful. How useful? Not much, actually, but there it is. What I did was clerical, and had no real relationship to any fighting anywhere on the planet. I consider myself fortunate to not have been faced with any moral or ethical decisions. It could have been otherwise.
I've often tried to put myself mentally in the cockpit of the B-17 my father flew, but am unable to visualize it clearly enough, despite having actually sat in the cockpit of one during a flight.