Because to anyone sane, if you are a gun-carrying criminal and you get shot by the police or another criminal; even if you are not actually pointing your gun at anyone at that precise moment, it's an occupational hazard, and nobody, not even the four children you left behind thanks to the reckless and selfish way you lived your life; has any right to utter a word of complaint.
It's funny (not ha ha) that everybody mentions the fact that Mr Duggan was a father of four, as though that makes it far more unreasonable that he was shot. Like punching someone in the face that wears glasses. Because now the police need to check the family status of any murderous criminals they tackle, just in case one of them has a fluffy kitten at home that depends on him.
I don't wish death on Mr Duggan. It would have been nice if he could have been rehabilitated (although I can't see why he would ever have had the motivation), but when I see that at least three INNOCENT people died in the riots as a direct result of Mr Duggan's criminal activity, and the misguided protests by his supporters, then I think that it's hard to work up any sympathy whatsoever for his family's loss.
Was Mr Duggan the kind of man who sold life-destroying drugs to children? We don't know. But the fact that he carried a gun suggests that he was not an entirely genial person. So what's the old saying? "Those who live by the sword..."