I don't think there's evidence of an "execution" either. But I also don't believe that Brown turned, taunted Wilson, and then charged him. I don't think that could happen in 3 seconds. Dorian Johnson said that "before he could get the second sentence out, he started shooting him". That matches the tape exactly. I believe the evidence shows that Wilson fired once from the car with Brown outside and Wilson inside. That shot would be JUSTIFIED, IMO. What Wilson did next is where he should be held accountable. He exited the vehicle and shot at the fleeing suspect. Even if you buy the eye socket story, I don't see how that helps, since a cop is not supposed to be shooting at fleeing suspects, but especially so when they themselves are running, and WOW, if his eyesight is blurry, what the hell would he be doing firing a weapon? This says to me his judgement was clouded. My suspicion is that he was pissed off beyond control.
What happened next is that one of the bullets hit Brown in the arm, which made him jerk and stop running. This matches all the accounts. Then he turned around and spoke to Wilson. This matches all accounts.
By this time, Brown had been shot twice, but neither was life threatening.
But as soon as Brown turns and speaks Wilson fires four more shots. We know this because the audio shows the time between the volleys is so short. There was no time to do much else. Had it been 10 seconds, maybe. But it's between 2-3 seconds. There's no way Brown stopped, turned, talked, then charged. Witnesses and the recordings and the forensics are all pretty well in agreement here unless you want to nitpick minor errors from the people who were there.
So that means all four final shots hit. This also shows the contrast between Wilson's shooting when the suspect is running vs. when he's standing still. All four of those shots hit. That suggests Brown wasn't moving very fast, and that Wilson wasn't running either.
Again, this matches the witnesses.
All in all, I think the evidence is squarely against the officer unless there's some hidden evidence the Ferguson police is sitting on for some unknown reason. But judging by the way they leak so far, I'd bet against that.