Shock value
Part of motivation behind terrorist acts is the shock they create. Barbaric and senseless acts are intended to state in the loudest way possible the presence and existence of these insurgents, their cause and their determination.
These people want to frighten the populace into complacency and keep those who would support coalition forces and the new Iraqi government from doing so. In that way, the brutal murder of someone who gave much of her life to the well-being of the people of Iraq must make awful sense to the insurgents. It does, indeed, further their goals.
One hopes that these awful human beings are caught and brought to justice for their crimes.