Scumbag or not he wasn't holding a gun when police shot him, how is this lawful?
Because the officer at the moment he pulled the trigger had an honestly held belief that Duggan was armed, and posed a risk to his life, the life of his colleagues, or the life of members of the public.
The jury have decided that the belief was mistaken, but that doesn't mean it wasn't honestly held.
The inquest report sums it up perfectly:
1) Did V53 honestly believe or may he honestly have believed, even if that
belief is mistaken, that at the time he fired the fatal shot, that he needed to use
force to defend himself or another; if your answer is NO then he cannot have
been acting in lawful self defence and you can put that issue to one side; if
your answer is YES then go on to consider:
2) Was the force used – the fatal shot – reasonable in all the circumstances?
Obviously if someone is under attack from someone he genuinely believes is
violent and armed – then that person cannot be expected to weigh up precisely
the amount of force needed to prevent that attack. But if he goes over top and
acts out of proportion to the threat then he would not be using reasonable force
and his action would be unlawful.