The difference between the killing of Margaret Hassan and the killing of the insurgent in the mosque
Both were brutal, senseless acts. The perpetrators of both deserve appropriate justice.
But Hassan's killers were anonymous, and can be portrayed by the people of Iraq as not being representative of them. Even what organization there is to the insurgency can probably claim that it was a splinter group. Foreigners, even, maybe.
The Marine was a representative of the occupying force - of the United States, of The West.
For this reason, I think that the mosque killing will do more harm to the occupying force than the Hassan killing will do to the insurgency.