War is a terrible thing, I don't think any rational person denies this. Civilized people have serious problems with it, on an abstract level at the very least. Make it concrete to them, by inundating them with pictures and video of the utter human devastation of war, and it could cause them to lose their will to support war even when necessary.
Refraining from showing the slaughter is therefore the responsible thing to do, because the only point to showing it to people who are perfectly aware of the evil of war would be to help bring about its end regardless of victory or loss. In short, there's a reason why the American media doesn't show an American soldier being blown to pieces by a roadside bomb, while a terrorist organization posts it on the Internet.