"The video shows a lapse in discipline."

That is exactly what neeeds to be determined. Discipline does break down under stress.

However there have been many occasions in many armies, including the British Army, when brutality towards captives and civilians has been connived at or encouraged by senior officers, and in such cases it is common practice to seek to present the events as merely "a lapse in discipline". "Abu Ghraib" was such a case, many (most?) people believe.

The important thing is to be certain that this latest incident is not another case of that happening. It is not something which can be taken as read, in Iraq anymore than in past conflicts in, for example, Northern Ireland or Kenya.