Under the marines own rules they're not supposed to kill them. Secondly this is not the only incident, on arab stations they are showing footage of three other incidents of the same nature, why these aren't being shown in the west I can't say.
These actions won't defeat the insurgants, instead it will recruit hundreds more.

There have also been reports of mass prisoner executions. Injured fighters being placed infront of tanks to be crushed.
While these stories are coming from refugees and are not yet confirmed, in a few months they might well be. Remeber Abu Graib, reports of torture came out months in advance of the pictures.