I don't know whether Mark Duggan was a "gangster". I don't think it's particularly of prime importance here. I don't think our position on this matter should be based on whether we think Mark Duggan was a good person.
Whether or not someone is a good person, has lead a moral life, etc., isn't a relevant consideration when it comes to the question of whether they should be executed by the police and whether, having been executed, the police should be properly held to account. What's the implication? "Duggan was involved in crime, he should've known it might end like this. Live by the sword, die by the sword"? Presumably that's not what you're saying, Matthew. Or is it "what the police did was wrong, but Duggan was a bad person so we shouldn't mourn him"?
I think the NCAFC BPC statement above is right to point out that the media does demonise young black men and attempt to stigmatise them by association with gang warfare and other crime. That's true, and a necessary point to make, regardless of whether or not the media portrayal of Duggan as a "gangster" has some truth to it.
Of course, demanding justice over what happened doesn't require us to prettify Duggan. I don't really know anything about him. I know a bit about the Noonans, and they sound a very nasty bunch indeed. But even if it was there was 100% iron-clad proof that Mark Duggan was a "gangster", the question of whether or not the police should be able to execute unarmed people with impunity would still very much remain.
The working-class left does need to think seriously about drugs, guns, and crime more generally, and how these things effect our communities. Possibly Mark Duggan was involved in those things. But we're not in a position to know that, and, as I say, I don't think our attitude to Mark Duggan's possible involvement in activity we think is harmful for working-class communities changes our attitude to the police and what they did.