People are jumping to conclusions (as usual). Some say the police officer was harassing a convenience store customer because he was shopping while black. Others say with certainty that the teen pointed a gun at the officer.
We have a video (shot from a great distance and inconclusive in my opinion). I'll put sound to it; any of these three versions are perfectly possible and as of now we have no idea which one is true.
Version 1: Teen comes out of the store talking to his friend: "I heard a voice telling me that cops are actually infiltrated aliens and they are out to kill all human beings. It is my duty to shoot them at first sight." Friend: "Man, I told you not to stop your antipsychotic medication. You need to go back to see your psychiatrist; you will get in trouble!" Cop comes out of his car and says "Good evening, nice young gentlemen. I saw you coming out of that store, and I'd like to ask you; do they sell donuts there?" Teen: "Get away, alien from outer space; you don't fool me; I'll kill you!" [Pulls gun, aims it at officer, gets shot, dies].
Version 2: Teen comes out of the store talking to his friend: "You know, I hate the police in this area. They seem to be racially profiling people like us." Cop comes out of his car and says "Good evening, gentlemen. We received a report of some trouble in this area and I'm patrolling. I'd like you to please tell me your intentions and whereabouts." Teen: "You motherf...r pig, I bet you're about to shoot me like you did to the guy in Ferguson just because I'm black; only, you are gay and a coward so I dare you to do it to me; I know a pig and chicken like you doesn't have it in your guts, I dare you to do it." Teen points his arm to cop (no gun) and taunts cop aggressively. Cop loses control, gets angry, shoots teen.
Version 3: Teen comes out of the store talking to his friend: "The weather is a bit cold tonight." Cop comes out of the car and says "That psychiatric medication I was taking is sh!t, I'm glad I tossed it in the garbage because now the voices in my head are telling me the truth that the two of you are infiltrated aliens I need to kill. Where are you going?" Teen says "Whoa, officer, calm down, I was just going to my cousin's home out there" (raises arm, no gun, points to the street where his cousin lives). Cop says "no, you are an outer space alien. There, I'll show you" (pulls gun, kills teen).
OK, I'm exaggerating a bit the psychotic guy who quit his psychiatric medication and is seeing space aliens around which is not very likely, just for the sake of the argument, but what I'm saying is that anything could have happened, from the cop talking calmly to the teen and getting threatened with a gun for no reason, to an incident fueled by the race tensions in that area getting out of hand again, from the cop being deranged and acting with completely unjustified force.
At this time we have no idea about what happened except that the police sustain that the teen pointed a gun at the cop, which may or may not be true (see post #66).
All other assumptions are premature until more facts are known.