A big question might be:
Was that the order they gave him or was it "turn around, put your hands up and drop the gun."
Apparently he did turn around and might have been in the process of complying with the rest and as he was raising his hands over his head the barrel of the gun inadvertently pointed in the direction of the police and that was all they needed to say they felt threatened and fill him full of lead.
Why didn't the police give him more time to comply with whatever their order was???
According to witnesses the teenager was carrying the gun in his left hand as he walked along the right side of the road. This would make it visible to passersby and to the police. If the command given to the teen was 'drop the gun' and rather than comply with that order the teenager turned to his right and brought the gun around with him the police would have less than a second to decide whether to shoot.
-btw If you'd like to see what happens when someone does decide to shoot at police with an AK47 just google "ak47 police video" there are plenty of examples.