Do the locals celebrate the murder of their own people?
I guess so, if they considered the dead collaborator collaborating with the American infidels.
Containing the various elements within Iraq really is like herding cats. If we set up defenses against roadside bombs, the insurgents assassinate and kidnap; when we set up for assassinations and kidnappings, they go back to bombing. We don't have enough troops in country to plug all the holes effectively.