I'm a Vet. of domestic wars but think this may help. For over 20 years I waded through the worst in Fire/EMS. For 25 years, one of my biggest hang ups was..."How can a loving God allow all this tragedy". For just as long, nobody could really answer this question. Not too long ago I finally got the answer, if one believes in "God".
God is not in control of everything. We have human will, acts of nature, sick and violent people, accidents....and war. He is not in "control" of everything no matter how devout one is. He can and does intervene at His will and not ours. Really that is the simple answer for me and for your Marine. This question tripped me up for many years and contributed to my PTSD. Believe in God or not ? God is not in control of all things....