he wasn't
yes they were specifically looking for him, yes they received information that he had a gun... (seemingly he was on his way to use that particular gun to avenge the death of one of his buddies)
if a police officer stops someone they know was carrying a gun and they react in any way other than being completely compliant and following all instructions given to them immediately then they're just putting themselves at risk
Except, they don't even need to be completely compliant to not be shot. My guess is the guy with the wooden leg, and this Duggan guy were shot prematurely for being a split second too hesitant to not responding to all the yelling, and of course a trigger happy policeman with a gun that he NEVER EVER normally gets to use.
Marksmen with guns on them as well as bullet proof vests, don't you think, they could actually wait 'til' they see a weapon before opening fire. I mean not too much to ask is it.. In either case, no gun was ever pulled on them.. Let assume for a second that they indeed do have weapons? Which they didn't (important FACT there). In the real world, they're going to have to take aim if they have any hope in actually hitting the cop that's shouting at them. Which neither did. That thing you see in movies, as in firing from the hip, almost never hits its target, let alone people protected in bullet proof vests. These people had guns on them. The person with the gun (most likely a rifle vs a handgun or shotgun) on them, is always going to lose the shootout, if they're going to need to take aim against the person who's got them squared away in their targets and finger on the trigger..
This should be NOTHING to do if Duggan was scum or not. This should be if this particular police officer was too itchy to pull the trigger or not.. He clearly was in this case. He should have given Duggan more opportunity to surrender, which he would have no doubt done, once he realised what exactly was going on. He's not going to take on an armed unit with cell phone is he.
That's all I'm trying to say.