The essential difference
Atrios puts it as succinctly as possible for those who are wondering why we express more outrage over Abu Ghraib than Nick Berg:
And, for the trolls - I am not responsible for things not done in my name. I am responsible for things that are. Please try to understand the difference.
When the monsters slice off the head of an innocent they harm the innocent and their loved ones. When the monsters flash smiles and thumbs up at the torture of Iraqi prisoners while wearing the uniform of my country and acting under the orders of my leadership then they are representing me in the eyes of the world.
There is no act the terrorists could perform that can cause as much harm to us as we can cause to ourselves.
The thugs who killed Nick Berg only sliced off his head. The people responsible for Abu Ghraib stabbed at the very heart of the American ideal. The former killed a man. The latter killed a dream.
Stop making excuses!