War crime is inexcusable but it is a by product of all conflicts and its perpetrators are not exclusively to be found on any one side or even only within official military machines, i.e. national armies, forces etc. These atrocities can just as easily be committed(and are)by so called freedom fighters, resistance movements and the like.

I know little about Dennis Boardman and I doubt if you do either. However, to describe him as a mere "killer" in such a condemnatory fashion is quite despicable. There is no evidence to suggest that he was anything other than a brave and honorourable man who was serving his country.
There is, of course, a distinction in law between "killing" and murder, in any case, and the former may be regarded as justifiable on occasion. Besides, not all servicemen have even actually "killed" another human being but many of them have gone through hell just the same including many of both the fallen and survivors.