I agree with what you say, but your example of context seems to (unconsciously?) reflect your own politics. What if the coworker's brother signed up specifically to go to Iraq because he truly believed in the mission? What if the coworker's family viewed him as an all American hero? In that case, why would the coworker be upset?

Your conversation leading to upset is based in the assumption that the coworker already shares your worldview in regard to the Iraq war. It's one I share too, so I can see why that'd be automatic, but I've met and known people who fought in Iraq. They didn't exactly seem ashamed!