I think it's important to understand that while it is unacceptable, troops who are suffering the stress of combat are not identically at fault as civilians who machine gun unarmed men. Military discipline demands that they be treated more harshly, but don't think of them in the same way.
It is also important to understand that, without excusing such behaviour, it is a habit of Al Qaeda influenced fighters to not surrender according to the conventions of war. It is very common for them to indicate surrender only to kill soldiers who attempt to take them into custody. As a consequence I think it is understandable that soldiers are unwilling to accept surrender. Illegal, and wrong, but understandable.