Insurgents or simply undercover British agents?

I've always wondered what benefit killing of the civilians in Iraq has for the Iraqi insurgents and why would they want to do so. I never really understood what the real reason for this could be. In fact, this would most likely benefit not the insurgents but the occupation forces. After all, it can easily create hostility in people towards the insurgents and as a result, it can potentially damage their cause. However, history; particularly in the Middle East, has repeatedly shown that imperialists in general and the Great Britain in particular have always used the "divide and rule" doctrine. Well, why not in Iraq? Why would they not try their dirty "tradition" in Iraq? How difficult, for an occupying power, would it be to blow up some cars packed with explosives and kill many, particularly Shias and blaim it on Sunnies? Perhaps not very difficult. They can easily do it, and perhaps benefit from it. The benefits? Well, as I said this can turn people against insurgency, and also it can easily create tension between Shias and Sunnies in Iraq and turn the fight against the occupation into an in-fight between the Iraqis. Who knows, may be this has been happening for quite some time and nobody noticed it, except this time when two British "Undercover Soldiers" were caught driving a booby-trapped car. Why would they need such car with such explosives? Why would they be dressed like Arabs (as they were)? Well, don't forget! Britain has done it before, may be with different methods matching the conditions and in different parts of the world but they have certainly done it before; and done it many times and have always benefitted from it. Their policies haven't changed, so there is no evidence indicating that their habit has changed.