Per reports: He was being tailed by the police. He spotted the police. He pointed a weapon at police. He got shot by the police. Happens all the time in the US, especially when the police have certainty that the person pointing a weapon at them is a gang banger.
If somebody looking like Duggan, with the dress and mannerisms of a gang-banger pointed a gun in my direction, I would have shot him without hesitation as well, and I'm a civilian.