The Inquest: Should the officer be charged?

Let me start by emphasizing that I am not a prosecutor and have never been one, except for a summer stint while in law school. But as a criminal defense lawyer, I have been involved in my share of murder cases. So I am coming from that angle.
I did not attend the recent Inquest. My information is based on what The Seattle Times and other publications printed. My understanding is that the officer saw this guy, stopped his car, ordered him to drop his knife, began approaching him and shot him four times, killing him. The victim appeared to have turned toward the officer but no witness described an aggressive move by the victim. Even the officer could only describe an aggessive "look." After all, only 4 seconds elapsed from the time he got out of his car until he shot him.
The Inquest jury of eight that heard this case was not unanimous. However on this key question: "Based on the available information at the time Officer Birk fired his weapon, did John T. Williams then pose an imminent threat of serious physical harm to Officer Birk?" Four jurors said no; three answered unknown and one said yes. This is the type of question a jury in a criminal case would ultimately answer: was this killing justified by the lawful use of force? (i.e, self defense.) My interpretation of the response to this question is that only one juror out of eight thought the killing was justified. According to the jury, they applied the beyond a reasonable doubt standard when they answered the questions posed. That is the same standard as a criminal case.
If a killing is not justified, shouldn't a jury of 12 decide if it is murder? I have handled several murder cases where we raised self defense because the other person, the victim, had started the fight. In one case, my client was punched several times and then in the scuffle drew a knife from his sock and began stabbing his opponent while they grappled. They were pulled apart and later the victim died. Two different juries found him guilty. (He won a second trial after a successful appeal.) Another case involved a man with a long record of violent crime who threatened to kill my client who ran into his house, got his gun and shot the victim 7-8 times from his porch, as the victim kept screaming "I'll kill you," though the victim had no weapon in his hand. In both cases, the juries felt that excessive force was used. They were instructed on the law which required that the lawful force employed be "not more than necessary." After the trials of those cases, the jurors all said my clients had a right to use self defense, but that they went too far.
The officer in the recent inquest went too far as well. He shot someone who was not an imminent threat four times. Wasn't that excessive force, or more force than necessary? If he were not a police officer, he would probably be prosecuted. However, the law gives him more protection. Under RCW 9A.16.040(3): "A public officer or peace officer shall not be held criminally liable for using deadly force without malice and with a good faith belief that such act is justifiable pursuant to this section." To be sure, most of the Inquest jurors agreed that he believed he was under an imminent threat. In other words, he was not acting in bad faith. What is more this law defines an imminent threat as "displays a weapon in a manner that could reasonably be construed as threatening." This statute will probably insulate the officer from prosecution.
The recent history of juries rejecting criminal prosecutions of police will also influence the prosecutor. Even where an officer is caught on video beating up a teenage girl, it shows how hard it is to get juries to convict the police of crimes. Of course, the officer in the Inquest case is likely to be sued, so there will be some civil justice.