There are two parts of Just War theory: ius ad bellum (which asks whether the war should be fought at all) and ius in bello (which asks whether the war, now determined to be just, is being fought justly). One clear violation of ius in bello is the practice of hostage-taking of non-combatants. It is, in fact, one of the things we have consistently deplored about the foaming Bronze Age Fanatics who make war upon our civilians (most notably on 9/11, but also in endless bombing and attacks on Israelis).
However, now that we are doing it, the WaPo calls it not “a clear violation of the rules of warfare” but “aggressive tactics”.
One of the corrupting things about war is that it tends to make the side with the just cause imitate the tactics of the enemy. We began WWII protesting indiscriminate bombing of civilian targets by the Nazis. We then outdid the Nazis at Dresden (one of the wickedest acts of the war). I tingle with anticipation for the justifications that will be offered in the blogosphere for hostage-taking.