The reason Brian is dead isn't actually that difficult Kathy. PTSD is why. What happened to Brian within his mind from his operational duty is the reason. The military are not to blame either, as Brian chose what he did. Brians choices unfortunately led to his death. It is no different from joining the military to begin with... being you do so knowing you could be deployed and shot at, killed. There are more killers than just being shot or blown up, PTSD is one of them.
Again, its not your fault, nor Jims, nor any others fault. It is merely a matter of circumstance that eventuated in Brian getting PTSD and what PTSD then helped him do. PTSD is a killer, I have never hid that from people. It is 100% true. It kills through behaviour, it kills through stress (heart attack, etc), it kills through depression.
I guess there is one way to help you understand just how much a killer it is....
lets take it to a poll shall we!