Well put, Pax.
Some additional observations. I don't know if your #2 against included the following but it bears expanding upon.
Ersland went past Parker (exposing his back to Parker) to chase the gunman down the street. At that point, the gunman had broken off the attack, and Ersland was acting aggressively.
Erlsand then comes back and walks past Parker. If Parker were acting in a manner that was threatening, one would have expected Ersland to have backed off or used the gun he had in hand. Unless it were empty. But if Ersland is counting the rounds used, he is a lot cooler in a gunfight than most cops, which isn't helpful to his defense.
Ersland gets a new gun and (instead of going to the back of the store to protect the female employees and perhaps exit the rear of the store or simply staying put and using counter as cover) goes back to confront Parker.
The video I saw wasn't all that clear, but the movements of Ersland appear to be that of a person who was going back to shoot Parker as opposed to someone who was checking on him and was startled by unexpected motion and shot in self defense.
Having said all that, I don't believe that Ersland is guilty of premeditated murder. There is no question in my mind that had Parker not come into the store with an accomplice to rob Ersland, none of this would have happened. Ersland over-reacted to a situation not of his own making, and I have no doubt that Ersland was greatly agitated. Killing in the "heat of passion" will reduce the degree of a homicide. The DA is correct to charge murder 1 since it should be up to a jury to decide what Ersland's state of mind was.
I don't put much stock in the theory that the DA is looking for political cover by charging murder 1. If the DA charges too low (here, say, manslaughter), the jury can not convict even if the evidence shows a higher level of offense. Simply put, you can't be convicted of a crime that you were not charged and tried for. Therefore, it is customary for DA's to charge the highest possible offense and let the jury sort it out.
We have a similar situation here in the SF Bay Area where a BART cop shot a young man, Oscar Grant, who was lying prone on the train platform. Grant was thought by the cops to have been in a fight on the BART train or otherwise being rowdy. The cop warned another cop that he was going to taser Grant but ended up pulling out his firearm and shooting Grant in the back and killing him. The DA here charged the cop with murder 1 too. And, yes, you could say that the DA did that for political reasons since the black community was up in arms over the shooting, but I think its SOP for most prosecutors.
No mention of race or creed or color or religion or anything else.