The truth is that the most startling thing about the brother's death is the complete lack of exposition of its circumstances. Nobody follows up, asks or even hints at what might have caused it. I think this is a nice move for Malick to pull because it gives the story an almost biblical sense. As a predominantly spiritual work, dwelling on details like how exactly a person dies are simply distracting. Like the "she's my sister" trick and locusts in Days of Heaven, it's clear that Malick is taking some cues from the Old Testament here. The Old Testament almost never goes into detail about the circumstances in which a person dies, it's much more interested in the spiritual lessons surrounding the death.
The lessons of the brother's death and how the family comes to grips with it are not meant to be contingent on any particular circumstance, they are meant to be universal and timeless. People die all the time, often in ways which we find unjust. The issue to deal with isn't how they died, it's how we relate to it. And by breaking our narrative expectations and hiding the cause of death, Malick draws our attention to this point.
Which isn't to say that I'm against speculating about what might have killed him, it's just that taken in context, it's clear that the film is intentionally drawing our attention away from that question.