Apparently, some still haven't gotten the memo how important it is to be the good guy who plays fair. Yeah, it would seem a no-brainer that all's fair in love and war, and it's a heck of a lot cheaper to go around assassinating the bad guys and the not-so-rational actors, the world public opinion be damned, right? Because in today's world, we have to assume that the word would get out that we did it.
And the answer is, so what, right? Just like with the torture at Abu Ghirab, who the hell cares, we're trying to save our fellow servicemembers' lives and win a war so shut the hell up and get out of the way, right?
Not so much. Even if torture were every bit as effective as some want to claim (and
it's not), actions like assassinations and torture have larger and deeper effects - all of which are bad for the nation which committed those crimes. A great example lay in Germany's invasion of Belgium in WWI.
Even until the day that Germany invaded Belgium on the way to France, England - the world's greatest superpower at the time - was officially neutral, and the majority of the population believed they would not go to war over Belgium despite England's treat obligation to do so if someone invaded Belgium. The Germans did invade on the assumption that the Belgians - with full knowledge that they couldn't hope to stop the Germans - would wisely step aside and let them through. The Belgians instead had a 'Gandalf moment' - "You shall not pass!" So the fit hit the shan...and the English declared war as a direct result.
At the time, the Germans - as in WWII - had a policy of collective punishment: if a partisan or sniper attacked the occupying German army or attempted to hinder its progress, they would kill a certain number of Belgian citizens from the closest town - sometimes just a few, sometimes a hundred or more - in order to teach the locals that war is not fair, leave us alone and we will let you live, try to hinder us, and we will make you regret it.
Many reports of the German Army's cruelty got out and were used to incredible effect by the French, American, and especially the British Empire's press. It was these reports of the German Army's atrocities that began to turn the attitude of the Americans against Germany, and the American government began conducting crucial trade deals much more favorably to England than to Germany. What's more, the stories of the Germans' atrocities were used to rally and inflame the passions of the Dominions, including Canada, Australia, New Zealand, India, and South Africa, allowing England to much more easily draw upon those populations for manpower and supplies.
In other words, it's not much of a stretch to say that their cruelty towards the Belgians was certainly one of the largest factors - if not the largest single factor - in Germany's loss of WWI. Their abhorrent actions turned the world against them, and gave their enemies wonderful recruiting tools, making it much easier to attract recruits (England's army was all-volunteer until about 1916) than it would have been otherwise.
THAT, sir, is the value of "playing fair" even in wartime - even if doing so costs you lives in the short run, maintaining the goodwill and sympathies of the rest of the world pays huge dividends in the long run.