"Why do people have to make such a fuss over the police assasinating a community leader? "
Because he was carrying an illegal firearm. Being a 'community leader' (which to be quite honest I have not read anywhere so I'd like to know where you got that from) doesn't allow you to do that.
Obviously we don't know the facts about the Duggan shooting yet, so we can't make calls on it. But I'm pretty sure I can say with confidence that even though it's far from unheard of for the police to kill civilians, police officers who use their firearms don't do it because they get off on killing people - which is what you'd think from the way they're described sometimes. Each time they've done it, it does seem to have been out of a genuine perception - sometimes wrong - that whoever they were targetting was a threat of some kind. As yet, we have no way of knowing whether the perception in this case was correct and it kind of depresses me that people refuse to see the ambiguities in the situation and immediately decide to side themselves against (or for) the police for no reason other than that's what they normally do.