Lawful orders, no. On the other hand if there are cases of servicemen and women executing civilians or other such war crimes (as we know happened in the last hotly contested conflict) then they're certainly wrong. I don't hold it against a Marine who shoots an insurgent because that's his job but in the end he's the one that has to live with the fact that he killed someone's father or brother or husband. Not his CO, not the joint chiefs, not Bush. He pulled the trigger, he's responsible for the death.
Justified or not it the responsibility for that death can't simply be passed along. I don't believe they're wrong in doing what they've sworn to do unless they're being asked to do things that are obviously illegal or that go against international conventions.
Though, with all due respect, I have a slightly different opinion on private contractors.
Challenge him how? Why does it seem right that he would have to engage in fisticuffs just to justify his beliefs?
I would gladly defend my words. I have my beliefs and despite the fact that they change and evolve as I gain life experience I am always willing to back up my beliefs. I do, however, believe it's wrong for someone to expect me to back up my beliefs in the form of a fight.
Granted I can hold my own against most people I meet I still don't see it as an intelligent way to go about things.
And no, I wouldn't have the right to try because trying to punch someone in the teeth for stating an opinion is considered assault. Threats of physical violence to get a point across is a sign of barbarism, not civility.